Sometimes I can't edit/remove software sources

[Juan Simón]

$ software-properties-gtk

(software-properties-gtk:21952): Gtk-WARNING **: Theme parsing error: gtk.css:55:40: Missing opening bracket in color definition
gpg: /tmp/tmpHEQRqG/trustdb.gpg: trustdb created
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py", line 842, 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'

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: software-properties-gtk 0.81.13.1
ProcVersionSignature: Ubuntu 3.0.0-14.23-core2 3.0.9-ck1
Uname: Linux 3.0.0-14-core2 x86_64
NonfreeKernelModules: nvidia
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
Date: Thu Dec 15 13:24:14 2011
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.