Comment 14 for bug 257639

I managed to re-create the error, although today it took me a few tries. I realized that perhaps I'm the cause of the problem. Specifically, the error occurs after the following steps:
1) In "Software Sources", enable a new source (e.g. ppa.launchpad development package), close, and reload.
2) Launch update manager (either manually or automatically) ---> new updates appear
3) Launch "Software Sources" again while update manager is open, but before installing the updates, disable the same source, close and reload
4) Error pops up.

Here is the CLI output:

:~$ apt-config dump | grep Update
APT::Periodic::Update-Package-Lists "1";
APT::Update "";
APT::Update::Post-Invoke-Success "";
APT::Update::Post-Invoke-Success:::: "touch /var/lib/apt/periodic/update-success-stamp 2>/dev/null || true";
APT::Update::Post-Invoke-Success:: "/usr/bin/dbus-send --system --dest=org.freedesktop.PackageKit --type=method_call /org/freedesktop/PackageKit org.freedesktop.PackageKit.StateHasChanged string:'cache-update'";

:~$ dpkg -l | grep packagekit
ii gstreamer0.10-packagekit 0.3.6-0ubuntu4 GStreamer plugin to install codecs using Pac
ii libpackagekit8 0.3.6-0ubuntu4 Library for accessing PackageKit
ii packagekit 0.3.6-0ubuntu4 provides a software installation daemon
ii packagekit-backend-apt 0.3.6-0ubuntu4 APT backend for packagekit

If the error occurs again in a different situation, I will be sure to note why.