I am getting the Error Message "Unhandled exception has occurred in your application. ........."

Clicking the Continue Button allows the Class Library Window to open. clicking Quit Button closes Phrogram.

 Details Button =

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at Phrogram.Controls.NamespaceBrowser.ProcessNamespaceNodeSelection(TreeNode node)
   at Phrogram.Controls.NamespaceBrowser.tvwNamespaces_AfterSelect(Object sender, TreeViewEventArgs e)
   at System.Windows.Forms.TreeView.OnAfterSelect(TreeViewEventArgs e)
   at System.Windows.Forms.TreeView.TvnSelected(NMTREEVIEW* nmtv)
   at System.Windows.Forms.TreeView.WmNotify(Message& m)
   at System.Windows.Forms.TreeView.WndProc(Message& m)
   at Phrogram.Controls.KplTreeView.WndProc(Message& message)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

A long list of Loaded Assemblies Follows and this message at the end:

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.

 What is JIT Debugging?