Confused by the behavior of Dispatcher.BeginInvoke()

Could someone shed some light on an issue I'm having?

I'm working on a wpf project. The scenario is as below:

I need to pop up a window(model window) on main UI thread and then close it. These works are started from another UI thread (to deter user from clicking on the main UI window.) then I close this window. The main code are displayed below. And it works.

As far as I know the close method would not get excuted before ShowDialog() returns (at least this is the case on UI thread, I mean code without dispatcher), does anyone have experience with multithread?

   Window window;
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        Thread thread = new Thread(() =>
           {


              //create a window and let user work from this thread

             //code is omitted.



               //create another window on main UI thread

              Application.Current.Dispatcher.BeginInvoke(new Action(() =>
                {
                    window = new Window();
                    window.ShowDialog();
                }));



               //do some work here

               Thread.Sleep(1000);

               Application.Current.Dispatcher.BeginInvoke(new Action(() =>
               {
                   //Thread.Sleep(1000);
                   window.Close();
               }));
           });

        thread.Start();
    }

Thank you for your time!