Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 352, in on_search_terms_changed
    self.update_navigation_button()
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 251, in update_navigation_button
    animate=True)
  File "/usr/share/software-center/softwarecenter/view/widgets/pathbar_gtk_atk.py", line 631, in add_with_id
    self.append(part, do_callback, animate)
  File "/usr/share/software-center/softwarecenter/view/widgets/pathbar_gtk_atk.py", line 394, in append
    self.set_active(part)
  File "/usr/share/software-center/softwarecenter/view/widgets/pathbar_gtk_atk.py", line 366, in set_active
    part.callback(self, part)
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 419, in on_navigation_search
    self.display_search()
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 370, in display_search
    self.emit("app-list-changed", len(self.app_view.get_model()))
TypeError: object of type 'NoneType' has no len()