How to destroy widget in GtK 3 python - python-2.7

Question

I would like to complete the following code in order to destroy window when I click on widget to close window.
#!/usr/bin/env python
from gi.repository import Gtk, Gio
import sys
class MyWindow(Gtk.ApplicationWindow):
def __init__(self, app):
Gtk.Window.__init__(self, title="Menubutton Example", application=app)
self.set_default_size(600, 400)
grid = Gtk.Grid()
menubutton = Gtk.MenuButton()
menubutton.set_size_request(80, 35)
grid.attach(menubutton, 0, 0, 1, 1)
menumodel = Gio.Menu()
menubutton.set_menu_model(menumodel)
menumodel.append("New", "app.new")
menumodel.append("Quit", "app.quit")
self.add(grid)
class MyApplication(Gtk.Application):
def __init__(self):
Gtk.Application.__init__(self)
def do_activate(self):
win = MyWindow(self)
win.show_all()
def do_startup(self):
Gtk.Application.do_startup(self)
new_action = Gio.SimpleAction.new("new", None)
new_action.connect("activate", self.new_callback)
self.add_action(new_action)
quit_action = Gio.SimpleAction.new("quit", None)
quit_action.connect("activate", self.quit_callback)
self.add_action(quit_action)
def new_callback(self, action, parameter):
print("You clicked \"New\"")
def quit_callback(self, action, parameter):
print("You clicked \"Quit\"")
self.quit()
app = MyApplication()
exit_status = app.run(sys.argv)
sys.exit(exit_status)
In case any problem code is here
http://learngtk.org/tutorials/python_gtk3_tutorial/html/application.html
When I clicked on the left top of window to destroy it, it's block and after I have the message is : Do you want to force application to leave or expect any answer?