private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (win2 == null)
            {
                win2       = new Window2();
                win2.Owner = this;
                win2.Show();

                win2.Closing += OnCloseEvent;
            }
        }
      private void Button_Click(object sender, RoutedEventArgs e)
      {
         if(win2 == null)
         {
            win2 = new Window2();
            win2.Owner = this;
            win2.Show();

            win2.Closing += OnCloseEvent;
            


         }
            
      }
 private void OnCloseEvent(object sender, EventArgs args)
 {
    win2.Closing -= OnCloseEvent;
    win2 = null;
 }
 private void OnCloseEvent(object sender, EventArgs args)
 {
     win2.Closing -= OnCloseEvent;
     win2          = null;
 }