i hope this answer will correct ur error
NullPointerExceptions are among the easier exceptions to diagnose, frequently. Whenever you get an exception in Java and you see the stack trace ( that's what your second quote-block is called, by the way ), you read from top to bottom. Often, you will see exceptions that start in Java library code or in native implementations methods, for diagnosis you can just skip past those until you see a code file that you wrote.
Then you like at the line indicated and look at each of the objects ( instantiated classes ) on that line -- one of them was not created and you tried to use it. You can start by looking up in your code to see if you called the constructor on that object. If you didn't, then that's your problem, you need to instantiate that object by calling new Classname( arguments ). Another frequent cause of NullPointerExceptions is accidentally declaring an object with local scope when there is an instance variable with the same name.
In your case, the exception occurred in your constructor for Workshop on line 75. <init> means the constructor for a class. If you look on that line in your code, you'll see the line
There are fairly clearly two objects on this line: denimjeansButton and this. this is synonymous with the class instance you are currently in and you're in the constructor, so it can't be this. denimjeansButton is your culprit. You never instantiated that object. Either remove the reference to the instance variable denimjeansButton or instantiate it.