Window replacement (Vala)

Window cannot be replaced by another window via button.

using GLib;
using Gtk;

public class AWE {

     public static int main (string [] args) {
          Gtk.init(ref args);
          new AWE();
          Gtk.main();
          return 0;
     }

     Gtk.Window main_window = new Gtk.Window();

     AWE () {
          X();
          main_window.set_default_size(200,200);
          main_window.set_position(Gtk.WindowPosition.CENTER);
          main_window.show_all();
          main_window.destroy.connect(Gtk.main_quit);
     }

     public void X () {
          main_window.title = "Test";
          var btn = new Gtk.Button.with_label("Click here");
          main_window.add(btn);

          btn.clicked.connect(() => {
               Z();
          });
     }

     public void Z () {
          var btn = new Gtk.Button.with_label("Click There");

          btn.clicked.connect(() => {
               X();
          });
          main_window.set_default(btn);

     }

}