[Gtk-sharp-list] A TreeStore problem

[jacocoder@o2.pl]

Hello!

I would like to ask about an error which occurs in my program. I have a TreeStore of two types:
string and MyContainer. MyContainer is a class used to represent objects in the tree.
When OnSelectionChanged event occurs I get an item and the object which is in the second column 
(MyContainer) of the iter. Then I call a method on this object. Everything is ok but after several repetitions
of this operation appears an error:

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object
in (unmanaged) (wrapper managed-to-native) Gtk.TreeStore:gtk_tree_model_get_value (intptr,Gtk.TreeIter&,int,GLib.Value&)
in <0x00004> (wrapper managed-to-native) Gtk.TreeStore:gtk_tree_model_get_value (intptr,Gtk.TreeIter&,int,GLib.Value&)
in <0x00029> Gtk.TreeStore:GetValue (Gtk.TreeIter,int,GLib.Value&)
in <0x00060> Gtk.TreeStore:GetValue (Gtk.TreeIter,int)
in <0x000bc> MyWindow:OnSelectionChanged (object,System.EventArgs)
in <0x00069> (wrapper delegate-invoke) System.MulticastDelegate:invoke_void_object_EventArgs (object,System.EventArgs)
in <0x0012d> GtkSharp.voidObjectSignal:voidObjectCallback (intptr,int)
in <0x0005a> (wrapper native-to-managed) GtkSharp.voidObjectSignal:voidObjectCallback (intptr,int)
in (unmanaged) (wrapper managed-to-native) Gtk.Application:gtk_main ()
in <0x00004> (wrapper managed-to-native) Gtk.Application:gtk_main ()
in <0x00007> Gtk.Application:Run ()
in <0x0002a> MainClass:Main (string[])

I do not understand why it does not happen at the first time? And what is the reason such an error occurs?

My project consists of three files: Main.cs, MyWindow.cs and MyContainer.cs. I use gtk-sharp
version 1.0.0.0 and mono 1.0.5. 
Source files are listed below.

I will be very grateful for any help.

Jakub Czeczotka

/* FILE Main.cs */
using System;
using Gtk;

class MainClass {

	public static void Main(string[] args) {
		Application.Init ();
		new MyWindow ();
		Application.Run ();
	}
}
/* END OF FILE Main.cs */

/* FILE MyWindow.cs */
using System;
using Gtk;
using GLib;

public class MyWindow : Window {

	private TreeView treeView = null;

	public MyWindow () : base ("MyWindow") {
		try {
			TreeStore store = new TreeStore (typeof(string), typeof(MyContainer));

			for (int i=0; i < 5; i++) {
				MyContainer _container = new MyContainer(i);
				TreeIter iter = store.AppendValues ("Demo " + i, _container);
			}

			Window win = new Window ("TreeView List Demo");
			win.DeleteEvent += new DeleteEventHandler (delete_cb);
			win.SetDefaultSize (400,250);

			ScrolledWindow sw = new ScrolledWindow ();
			win.Add (sw);

			treeView = new TreeView ();
			treeView.Model = store;
			treeView.HeadersVisible = true;
			treeView.Selection.Changed += new EventHandler(this.OnSelectionChanged);

			treeView.AppendColumn ("Demo", new CellRendererText (), "text", 0);

			sw.Add (treeView);
			sw.Show ();
			win.ShowAll ();
		} catch (Exception ex) {
			Console.WriteLine(ex.ToString());
		}
	}

	void OnMyWindowDelete (object o, DeleteEventArgs args) {
		Application.Quit ();
	}

	private void OnSelectionChanged (object o, EventArgs args) {
		TreeIter iter;
		TreeModel model;

		if (((TreeSelection)o).GetSelected (out model, out iter)) {
			object val = treeView.Model.GetValue(iter, 1);
			if (null != val) {
				Console.WriteLine ("{0} <- was selected", val.ToString());
				MyContainer mc = val as MyContainer;
				mc.Show();
			}
		}
	}

	private void delete_cb (System.Object o, DeleteEventArgs args) {
		Application.Quit ();
		args.RetVal = true;
	}
}
/* END OF FILE MyWindow.cs */

/* FILE MyContainer.cs */
using System;
using Gtk;

public class MyContainer : GLib.Object {
	static GLib.GType gtype;

	public static new GLib.GType GType {
		get {
			if (gtype == GLib.GType.Invalid)
				gtype = RegisterGType (typeof (MyContainer));
			return gtype;
		}
	}

	public MyContainer() : base (GType) {
	}

	public MyContainer(int _value) : base (GType) {
		this._value = _value;
	}

	public override string ToString() {
		return "my value is " + this._value.ToString();
	}

	public void Show () {
		Console.WriteLine ("My private value is " + _value.ToString());
	}

	private int _value = 0;
}
/* END OF FILE MyContainer.cs */