After deleteing TextBox'es application losing focus.

Platform : .
NET , Framework 1.1, VS 2003
Example scenario :
I have a Panel with few TextBox'es on it.
On Leave event I'm trying to delete all controls on the
Panel
and add new controls. Of course before deleting I deatached
lost focus event.
Problem ( It only occure when user use mouse to navigate ! There is no
problem when use TAB key. ) :
- Application losing focus and user can't close it !
- Sometimes NullException occure
Complete code :

using System;
using System.Windows.Forms;

namespace WindowsApplication3
{
public class Form1 : System.Windows.Forms.Form
{
private System.ComponentModel.Container components = null;

public Form1() {
InitializeComponent();
AddControls();
}

protected override void Dispose( bool disposing ) {
if( disposing ) {
if (components != null) {
components.Dispose();
}
}
base.Dispose( disposing );
}

private void InitializeComponent() {
this.components = new System.ComponentModel.Container();
this.Size = new System.Drawing.Size(300,300);
this.Text = "Form1";
}

  [STAThread]

static void Main() {
Application.Run(new Form1());
}

  private void AddControls() {
     for ( int i = 0; i < 2; i++) {
        TextBox tb = new TextBox();
        tb.Dock = DockStyle.Top;
        tb.Leave += new EventHandler(tb_Leave);
        this.Controls.Add( tb );
     }
  }

  private void tb_Leave(object sender, EventArgs e) {
     foreach ( TextBox tb in this.Controls ) {
        tb.Leave -= new EventHandler(tb_Leave);
     }
     this.Controls.Clear();
     AddControls();
  }

}
}