So far it looks pretty good, except in the GUI I can not get the Modify, Save, Delete, Add and Search buttons to work. They produce error codes :
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javaapplication54.ModifyButtonHandler.actionPerformed(Inventory6.java:488)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6041)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5806)
at java.awt.Container.processEvent(Container.java:2058)
at java.awt.Component.dispatchEventImpl(Component.java:4413)
at java.awt.Container.dispatchEventImpl(Container.java:2116)
at java.awt.Component.dispatchEvent(Component.java:4243)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
at java.awt.Container.dispatchEventImpl(Container.java:2102)
at java.awt.Window.dispatchEventImpl(Window.java:2440)
at java.awt.Component.dispatchEvent(Component.java:4243)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

package javaapplication54;

import java.awt.*;  // import the java awt package
import java.awt.event.*;  // Import the java event package
import javax.swing.*; // Import the java swing package
import java.util.*; // Import the java utility package
import java.io.*; // Import the java input output package
import java.awt.Image;
import javax.swing.ImageIcon;





public class Inventory6 {

    // main method begins
    public static void main(String[] args) {
      
      

        Movie dvd = null;
        Inventory inventory = new Inventory();        

        dvd = new Movie(2356, "Legally Blonde", 25, 14.99f, "G");
        System.out.println(dvd);
        inventory.addMovie(dvd);                    
        
        dvd = new Movie(5684, "National Tressure", 3, 12.00f, "G");
        System.out.println(dvd);
        inventory.addMovie(dvd);                    
        
        dvd = new Movie(5564, "Sweet Home Alabama", 15, 15.75f, "G");
        System.out.println(dvd);
        inventory.addMovie(dvd);                    

        dvd = new Movie(5562, "Shrek", 7, 9.99f, "G");
        inventory.addMovie(dvd);                    
        System.out.println(dvd);

      
                          

        inventory.printInventory();
          new InventoryGUI(inventory);
          
    } // end main

} // end Inventory6






// Extends DVD class from the base class DVD
class DVD {
    private int itemNo;
    private String title;
    private int inStock;
    private float unitPrice;

    DVD() {
        
    }

    DVD(int itemNo, String title, int inStock, float unitPrice) {
        this.itemNo    = itemNo;
        this.title     = title;
        this.inStock   = inStock;
        this.unitPrice = unitPrice;
      
    }

    public int getItemNo()      { return itemNo; }
    public String getTitle()    { return title; }
    public int getInStock()     { return inStock; }
    public float getUnitPrice() { return unitPrice; }
  
    public float value() {
        return inStock * unitPrice;
    }

    @Override
    public String toString() {
        return String.format("itemNo=%2d   title=%-22s   inStock=%3d   price=$%7.2f   value=$%8.2f",
                              itemNo, title, inStock, unitPrice, value());
    }

    void addDVD() {
       
    }

    void closeFile() {
        
    }

    void openFile() {
        
    }

} // end DVD
class Inventory {
    private static int itemNo;

    static int getitemNo() {
         return itemNo;
         
    }
    // Setup an array of Movies
    private final int INVENTORY_SIZE = 100;
    private DVD[] items;
    private int numItems;
    
    Inventory() {
        items = new Movie[INVENTORY_SIZE];
        numItems = 0;
    }

    public int getNumItems() {
        return numItems;
    }

    public DVD getDVD(int n) {
        return items[n];
    }

    // Adds a Movie to the array of Movies. Adds to first empty slot found.
    public void addMovie(DVD item) {
        items[numItems] = item;    
        ++numItems;
    }

    
    public double value() {
        double sumOfInventory = 0.0;

        for (int i = 0; i < numItems; i++)
            sumOfInventory += items[i].value();
                        
        return sumOfInventory;
    }

    
    public void printInventory() {
        System.out.println("\n DVD Inventory\n");
      
        
        if (numItems <= 0) {
                   } else {
            for (int i = 0; i < numItems; i++)
                System.out.printf("%3d   %s\n", i, items[i]);
            System.out.printf("\nTotal value is $%,.2f\n\n", value());
        }
    }
    
} // end Inventory
class Movie extends DVD {
    // Holds movie Rating and adds restocking fee
    private String movieRating;

    // Constructor, calls the constructor of Movie first
    public Movie(int MovieID, String itemName, int units, float itemPrice, String Rating) {
        super(MovieID, itemName, units, itemPrice);    
        // Pass on the values needed for creating the Movie class first thing
        this.movieRating = Rating;        
    }

    



    // To set the rating manually
    public void setRating(String Rating) {
        movieRating = Rating;
    }

    // Get the rating
    public String getRating() {
        return movieRating;
    }

    // Overrides value() in Movie class by calling the base class version and
    // adding a 5% restocking fee on top
    @Override
     public float value() {
        return super.value() + restockingFee();
    }

    // Simply gets the base class's value, and figures out the 5% restocking fee only
    public float restockingFee() {
        return super.value() * 0.05f;
    }

} // end Movie
class ArcsJPanel extends JPanel
{
    
    @Override
    public void paintComponent( Graphics g )
    {
        super.paintComponent( g ); // call superclass's paintComponent

        // draws larger 3d rectangle
        g.setColor( Color.RED );
        g.draw3DRect( 45, 15, 180, 40, true);
        
        // draws small black square
        g.setColor( Color.BLACK );
        g.draw3DRect( 5, 15, 40, 40, true );
        g.fill3DRect( 5, 15, 40, 40, false );
        
        //draws circle
        g.setColor( Color.RED );
        g.drawArc( 5, 15, 40, 40, 10, 360 );
        
        //fills circle
        g.setColor( Color.WHITE);
        g.fillArc( 5, 15, 40, 40, 10, 360 );

        // draws company name
        g.setColor( Color.BLACK );
        g.setFont( new Font( "Serif", Font.BOLD, 25 ) );
        g.drawString( "Estey's DVD Shack.", 10, 45 );

     } // end method paintComponent

} // end class ArcsJPanel


// GUI for the Inventory
// Contains an inventory of DVD's and lets the user step through them one by one
class InventoryGUI extends JFrame
{
    // access inventory for DVD Collection
    private Inventory theInventory;
    
    // index in the inventory of the currently displayed DVD.
    // the index starts at 0, goes to the number of DVDs in the inventory minus 1
    private int index = 0;
    
    // GUI elements to display currently selected DVD information
    private final JLabel itemNumberLabel = new JLabel("  Item Number:");
    private JTextField itemNumberText;
    
    private final JLabel prodnameLabel = new JLabel("  Product Name:");
    private JTextField prodnameText;
    
    private final JLabel prodpriceLabel = new JLabel("  Price:");
    private JTextField prodpriceText;
    
    private final JLabel numinstockLabel = new JLabel("  Number in Stock:");
    private JTextField numinstockText;
    
    private final JLabel valueLabel = new JLabel("  Value:");
    private JTextField valueText;
    
    private final JLabel movieRatingLabel = new JLabel("  Rated:");
    private JTextField movieRatingText;
    
    private final JLabel totalValueLabel = new JLabel("  Inventory Total Value (5% restock fee included:");
    private JTextField totalValueText;
    
  
    
    
      
    

    

    private JPanel centerPanel;
    private JPanel buttonPanel;





    // constructor for the GUI, in charge of creating all GUI elements
    InventoryGUI(Inventory inventory) {
        super("Movie Inventory");
        final Dimension dim = new Dimension(140, 20);
        final FlowLayout flo = new FlowLayout(FlowLayout.LEFT);
        JPanel jp;

        // create the inventory object that will hold the product information
        theInventory = inventory;        
        
      
        centerPanel = new JPanel();
        centerPanel.setLayout(new BoxLayout(centerPanel, BoxLayout.Y_AXIS));

      
        
       buttonPanel = new JPanel();
        JButton firstButton = new JButton("First");    
        firstButton.addActionListener((ActionListener) new firstButtonHandler());
        buttonPanel.add(firstButton);

JButton previousButton = new JButton("Previous");
previousButton.addActionListener(new previousButtonHandler());
buttonPanel.add(previousButton);

JButton nextButton = new JButton("Next");
nextButton.addActionListener(new NextButtonHandler());
buttonPanel.add(nextButton);

JButton lastButton = new JButton("Last");
lastButton.addActionListener(new lastButtonHandler());
buttonPanel.add(lastButton);

JButton addButton = new JButton("Add");
addButton.addActionListener(new AddButtonHandler());
buttonPanel.add(addButton);

JButton deleteButton = new JButton("Delete");
deleteButton.addActionListener(new DeleteButtonHandler());
buttonPanel.add(deleteButton);

JButton modifyButton = new JButton("Modify");
modifyButton.addActionListener(new ModifyButtonHandler());
buttonPanel.add(modifyButton);

JButton saveButton = new JButton("Save");
saveButton.addActionListener(new SaveButtonHandler());
buttonPanel.add(saveButton);

JButton searchButton = new JButton("Search");
searchButton.addActionListener(new SearchButtonHandler());
buttonPanel.add(searchButton);

   centerPanel.add(buttonPanel);
            
        jp = new JPanel(flo);
        itemNumberLabel.setPreferredSize(dim);
        jp.add(itemNumberLabel);
        itemNumberText = new JTextField(3);
        itemNumberText.setEditable(false);
        jp.add(itemNumberText);
        centerPanel.add(jp);
        
        jp = new JPanel(flo);
        prodnameLabel.setPreferredSize(dim);
        jp.add(prodnameLabel);
        prodnameText = new JTextField(17);
        prodnameText.setEditable(false);
        jp.add(prodnameText);
        centerPanel.add(jp);
        
        jp = new JPanel(flo);
        movieRatingLabel.setPreferredSize(dim);
        jp.add(movieRatingLabel);
        movieRatingText = new JTextField(17);
        movieRatingText.setEditable(false);
        jp.add(movieRatingText);
        centerPanel.add(jp);
        
        jp = new JPanel(flo);
        prodpriceLabel.setPreferredSize(dim);
        jp.add(prodpriceLabel);
        prodpriceText = new JTextField(17);
        prodpriceText.setEditable(false);
        jp.add(prodpriceText);
        centerPanel.add(jp);
        
        jp = new JPanel(flo);
        numinstockLabel.setPreferredSize(dim);
        jp.add(numinstockLabel);
        numinstockText = new JTextField(5);
        numinstockText.setEditable(false);
        jp.add(numinstockText);  
        centerPanel.add(jp);
        
        
        
        jp = new JPanel(flo);
        valueLabel.setPreferredSize(dim);
        jp.add(valueLabel);
        valueText = new JTextField(17);
        valueText.setEditable(false);
        jp.add(valueText);
        centerPanel.add(jp);
        
        // add the overall inventory information to the panel
        jp = new JPanel(flo);
        totalValueLabel.setPreferredSize(dim);
        jp.add(totalValueLabel);
        totalValueText = new JTextField(17);
        totalValueText.setEditable(false);
        jp.add(totalValueText);
        centerPanel.add(jp);

        // add the panel to the GUI display
        setContentPane(centerPanel);

        repaintGUI();

        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(550,500);
        setResizable(true);
        setLocationRelativeTo(null);
        setVisible(true);
    }

    // (re)display the GUI with current product's information
    public void repaintGUI() {
        Movie temp = (Movie) theInventory.getDVD(index);
        
        if (temp != null) {
            itemNumberText.setText("" + temp.getItemNo());    
            prodnameText.setText(temp.getTitle());
            prodpriceText.setText(String.format("$%.2f", temp.getUnitPrice()));
            movieRatingText.setText(temp.getRating());
            numinstockText.setText("" + temp.getInStock());
            valueText.setText(String.format("$%.2f", temp.value()));
        }
        totalValueText.setText(String.format("$%.2f", theInventory.value()));
    }

    class NextButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            int numItems = theInventory.getNumItems();
            index = (++index) % numItems;
            repaintGUI();
            
        }
    }
   class previousButtonHandler implements ActionListener{
        public void actionPerformed (ActionEvent e){
            index--;
            if(index < 0) {
               index = theInventory.getNumItems() - 1;
            }
            repaintGUI();

         }
    }
    class firstButtonHandler implements ActionListener{
        public void actionPerformed (ActionEvent e){
           index = theInventory.getNumItems() - 4;
            repaintGUI();
            
        }
        
    }
   class lastButtonHandler implements ActionListener{
        public void actionPerformed (ActionEvent e){
            index = theInventory.getNumItems() - 1;
            repaintGUI();
        }

  
    }
class AddButtonHandler implements ActionListener {
public void actionPerformed(ActionEvent e){
repaintGUI();
}
}
class SaveButtonHandler implements ActionListener {
public void actionPerformed(ActionEvent e){
    int itemNo = Inventory.getitemNo();
            DVD dvd = new DVD(); //calls inventroy storage class
            dvd.openFile();
            int currentRecord = 0; // keeps track of number of cycles

            do // cycles through the list adding them one at a time
            {
                dvd.addDVD();
                currentRecord = currentRecord + 1;
                index = (++index) % itemNo;

            } while (currentRecord < itemNo); //ends while

                dvd.closeFile(); //closes file
        }//end action

    } // end class


}
class ModifyButtonHandler implements ActionListener {
    private Object prodName;
    private Object itemNo;
    private Object numinstockText;
public void actionPerformed(ActionEvent e){
if (prodName.getClass().equals(""))  //traps for blank entry
            {
                JOptionPane.showMessageDialog(null, "Please Complete the Entry.","Negative GhostRider the pattern is full", JOptionPane.ERROR_MESSAGE);
                repaintGUI();
            }


            if (itemNo.getClass().equals(""))  //traps for blank entry
            {
                JOptionPane.showMessageDialog(null, "Please Complete the Entry.","Negative GhostRider the pattern is full", JOptionPane.ERROR_MESSAGE);
                repaintGUI();
            }

            try // traps for letters and blank entry
            {
            
                
                
                
                }
                catch (Exception d)
                {
                JOptionPane.showMessageDialog(null, "Recheck Entry use numbers for price and quantity.","Negative GhostRider the pattern is full", JOptionPane.ERROR_MESSAGE);
                repaintGUI();
                }


            String name; int number; double amount, price; String title; // declares variables
            
                          name = prodNameText();
            number = itemNumberText();
            amount = numinstockText();
            price = prodpriceText();
        
        

        

            
            
            repaintGUI();
        }

    private int itemNumberText() {
        return itemNumberText();
    }

    private double numinstockText() {
        return numinstockText();
    }

    private String prodNameText() {
        return prodNameText();
    }

    private double prodpriceText() {
        return prodpriceText();
    }

    private void repaintGUI() {
       repaintGUI();
    }

    } // end class
                    
                    



class DeleteButtonHandler implements ActionListener {
      
public void actionPerformed(ActionEvent e){
repaintGUI();

}

    private void repaintGUI() {
       repaintGUI();
    }
}



class SearchButtonHandler implements ActionListener {
    private Object searchText;

    
public void actionPerformed(ActionEvent e,String prodNameText,String search){
    

            String name = prodNameText();
            int number = itemNumberText();
            int units = numinstockText();
            int currentDVD = 0;
    
            do // gets text and checks for a match
            {    
                
                repaintGUI();
                name = prodNameText();
                number = itemNumberText();
                
            

                if(name.equalsIgnoreCase( search )) // stops on item
                    repaintGUI();

                

                
        

            }while(name.equalsIgnoreCase(search ) != true); //ends do while
      

        }//end action
    private int itemNumberText() {
       return itemNumberText();
       
    }

    private int numinstockText() {
        return numinstockText();
    }
    private String prodNameText() {
        return prodNameText();
    }

    private void repaintGUI() {
        
    }

    public void actionPerformed(ActionEvent e) {
       
    }

      
    } // end class
  
class InventoryStorage // creates the output file
{
    private Formatter output; // object used to output text to file

        public void openFile()
        {
            try
            {
                    String strDirectoy ="C://data/";

                    // Create one directory
                    boolean success = (new File(strDirectoy)).mkdir();
                    
                if (success)
                {

                          JOptionPane.showMessageDialog(null, "we had to create a directory named data in your C drive.","That's affirmative", JOptionPane.PLAIN_MESSAGE);

                   }   //end if


             }//end try

            catch (Exception e)
            {
              
                
                      JOptionPane.showMessageDialog(null, "You do not have write access to the C: drive.","Negative GhostRider the pattern is full", JOptionPane.ERROR_MESSAGE);

                }//end catch
  


            try
            {
                
                          output = new Formatter( "C:/data/inventory.dat");

            } // end try

        catch ( SecurityException securityException ) //catches for write access and exits program
        {
            JOptionPane.showMessageDialog(null, "You do not have write access to this file.","Negative GhostRider the pattern is full", JOptionPane.ERROR_MESSAGE);
            System.exit( 1 );

        } // end catch

        catch ( FileNotFoundException filesNotFoundException ) //catches for write access and exits program
        {
            JOptionPane.showMessageDialog(null, "Please create a file named data in your c drive","Negative GhostRider the pattern is full", JOptionPane.ERROR_MESSAGE);
            System.exit( 1 );

        } // end catch


        JOptionPane.showMessageDialog(null, "file saved successfully.","That's affirmative", JOptionPane.PLAIN_MESSAGE);



    } // end method openFile

        


    
    public void closeFile() // close the file
    {
        if ( output != null )
        output.close();

    } // end method closeFile

}// end InventoryStorage class

The error is occurring on line 488 and is a NullPointerException, which means that something is being referenced on line 488 that hasn't been initialised properly. Which line of code is number 488 (in Inventory.java file)?