Comment 4 for bug 820150

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py", line 839, in on_remove_clicked
    self.backend.RemoveSource(str(source_entry))
  File "/usr/lib/python2.7/dist-packages/dbus/proxies.py", line 143, in __call__
    **keywords)
  File "/usr/lib/python2.7/dist-packages/dbus/connection.py", line 630, in call_blocking
    message, timeout)
dbus.exceptions.DBusException: org.freedesktop.DBus.Python.AttributeError: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/dbus/service.py", line 702, in _message_cb
    retval = candidate_method(self, *args, **keywords)
  File "/usr/lib/python2.7/dist-packages/softwareproperties/dbus/SoftwarePropertiesDBus.py", line 269, in RemoveSource
    self.remove_source(source.encode("utf-8"))
  File "/usr/lib/python2.7/dist-packages/softwareproperties/SoftwareProperties.py", line 711, in remove_source
    if source.file != apt_pkg.config.find_file("Dir::Etc::sourcelist"):
AttributeError: 'NoneType' object has no attribute 'file'