Problem in SMS Bomber

[sweetifffy]

Hi
I have made my software of sms bomber in Visual Basic 2008 . When i open software and enter my password and then i write my number (for testing ) and click on bomb it sent only 5 or 6 sms and then it stops and this error occured
" Unhandled exception has occured in your application"

See the end of this message for details on invoking 
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at NewBomber.Form1.Button2_Click(Object sender, EventArgs e)
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.PerformClick()
   at NewBomber.Form1.Timer2_Tick(Object sender, EventArgs e)
   at System.Windows.Forms.Timer.OnTick(EventArgs e)
   at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)


************** Loaded Assemblies **************
mscorlib
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
    CodeBase: file:///C:/WINDOWS/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll
----------------------------------------
NewBomber
    Assembly Version: 1.0.0.0
    Win32 Version: 1.0.0.0
    CodeBase: file:///C:/Documents%20and%20Settings/Administrator/Local%20Settings/Apps/2.0/1ZZZWV9L.5BH/9J1982ME.4WJ/newb..tion_ef5720d3fd82f315_0001.0000_a65f56cd24d50f0b/NewBomber.exe
----------------------------------------
Microsoft.VisualBasic
    Assembly Version: 8.0.0.0
    Win32 Version: 8.0.50727.1433 (REDBITS.050727-1400)
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/Microsoft.VisualBasic/8.0.0.0__b03f5f7f11d50a3a/Microsoft.VisualBasic.dll
----------------------------------------
System
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll
----------------------------------------
System.Windows.Forms
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
----------------------------------------
System.Drawing
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
----------------------------------------
System.Runtime.Remoting
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Runtime.Remoting/2.0.0.0__b77a5c561934e089/System.Runtime.Remoting.dll
----------------------------------------

************** JIT Debugging **************
To enable just-in-time (JIT) debugging, the .config file for this
application or computer (machine.config) must have the
jitDebugging value set in the system.windows.forms section.
The application must also be compiled with debugging
enabled.

For example:

<configuration>
    <system.windows.forms jitDebugging="true" />
</configuration>

When JIT debugging is enabled, any unhandled exception
will be sent to the JIT debugger registered on the computer
rather than be handled by this dialog box.

The Visual Basic c# code is

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If TextBox1.Text.Contains("abcdef") Then
            WebBrowser1.Navigate("http://waridtijarat.waridtel.com/session/signup")
            TextBox1.Visible = False
            Button1.Visible = False
            Label1.Visible = False
            Label3.Visible = True
            TextBox2.Visible = True
            Button2.Visible = True
            Button3.Visible = True
            ProgressBar1.Visible = True
        Else
            MsgBox("Wrong Password. Get Password from Baber Khan")
        End If
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        ProgressBar1.Value.Equals("0")
        ProgressBar2.Value.Equals("0")
        WebBrowser1.Document.GetElementById("user[mobile_number]").SetAttribute("value", TextBox2.Text)
        WebBrowser1.Document.GetElementById("commit").InvokeMember("click")
        Timer1.Start()
        Label4.Text = Label4.Text + 1
    End Sub

    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        If ProgressBar1.Value = ProgressBar1.Maximum Then
            WebBrowser1.GoBack()
            Timer2.Start()

        End If
    End Sub

    Private Sub WebBrowser1_ProgressChanged(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserProgressChangedEventArgs) Handles WebBrowser1.ProgressChanged
        ProgressBar1.Value = e.CurrentProgress
        ProgressBar2.Value = e.CurrentProgress
        ProgressBar1.Value.Equals("0")
        ProgressBar2.Value.Equals("0")

    End Sub

    Private Sub Timer2_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer2.Tick
        If ProgressBar2.Value = ProgressBar2.Maximum Then
            Button2.PerformClick()
        End If
    End Sub
End Class

The Problem is in this line
WebBrowser1.Document.GetElementById("user[mobile_number]").SetAttribute("value", TextBox2.Text)