为什么这个PyGTK菜单是空的？

import gtk

def on_menu_item_activate(widget):
    print("Menu item activated")

def create_menu():
    menu = gtk.Menu()

    menu_item = gtk.MenuItem("菜单项")
    menu_item.connect("activate", on_menu_item_activate)
    menu.append(menu_item)

    return menu

def create_window():
    window = gtk.Window()
    window.set_default_size(200, 200)

    vbox = gtk.VBox()
    window.add(vbox)

    menubar = gtk.MenuBar()
    vbox.pack_start(menubar, False, False, 0)

    menu = create_menu()
    menubar.append(menu)

    return window

def main():
    window = create_window()
    window.connect("destroy", gtk.main_quit)
    window.show_all()
    gtk.main()

if __name__ == "__main__":
    main()