title: AttributeError: 'NoneType' object has no attribute 'append_text'
body: <pre><code>#!/usr/bin/python
  # -*- coding: utf-8 -*-
  import gi
  gi.require_version('Gtk', '3.0')
  from gi.repository import Gtk
  class Handler:
      def button_is_clicked(self, button):
          ouraboutwindow.run()
          ouraboutwindow.hide()
      def enter_button_clicked(self, button):
          print ourentry.get_text() + ourcomboboxtext.get_active_text()
  builder = Gtk.Builder()
  builder.add_from_file("ui.glade")
  builder.connect_signals(Handler())
  ournewbutton = builder.get_object("button1")
  window = builder.get_object("window1")
  ourcomboboxtext = builder.get_object("comboboxtext1")
  default_text = [" World ", " Earth ", " All "]
  for x in default_text:
      ourcomboboxtext.append_text(x)
  ourcomboboxtext.set_active(0)
  ourentry = builder.get_object("entry1")
  ourentry.set_max_length(15)
  ourentry.set_placeholder_text("Enter A Text Here..")
  ouraboutwindow = builder.get_object("aboutdialog1")
  window.connect("delete-event", Gtk.main_quit)
  window.show_all()
  Gtk.main
  </code></pre>
  
  <p>I am trying to learn the widgets in glade interface where in i have created a text entry a combobox and a button. But the combobox is giving me some error. </p>
  
  <p>Here i will type something in text entry then select something from combobox and together it will be printed in the shell.
  Please help.</p>