Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 397, in on_category_activated
    self.set_category(category)
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 421, in set_category
    self.refresh_apps()
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 186, in refresh_apps
    self._refresh_apps_with_apt_cache()
  File "/usr/share/software-center/softwarecenter/view/softwarepane.py", line 56, in wrapper
    f(*args, **kwargs)
  File "/usr/share/software-center/softwarecenter/view/availablepane.py", line 197, in _refresh_apps_with_apt_cache
    old_model.active = False
AttributeError: 'NoneType' object has no attribute 'active'