update-manager crashed with UnboundLocalError in show_diff(): local variable 'line_number' referenced before assignment

[Kamil Pajak]

While updating Steam, Software Updater crashed:
***
"Replace your changes in '/etc/apt/sources.list.d/steam.list' with a later version of the configuration file? If you don't know why the file is there already, it is usually safe to replace it."
***
Clicking on "Keep" or "Replace" does not change anything (Screenshot)

ProblemType: Crash
DistroRelease: Ubuntu 13.04
Package: update-manager 1:0.181
ProcVersionSignature: Ubuntu 3.8.0-4.9-generic 3.8.0-rc6
Uname: Linux 3.8.0-4-generic i686
ApportVersion: 2.8-0ubuntu4
Aptdaemon:

Architecture: i386
Date: Fri Feb 8 13:43:21 2013
ExecutablePath: /usr/bin/update-manager
GsettingsChanges:
 b'com.ubuntu.update-manager' b'first-run' b'false'
 b'com.ubuntu.update-manager' b'launch-time' b'1360327262'
 b'com.ubuntu.update-manager' b'show-details' b'true'
 b'com.ubuntu.update-manager' b'window-height' b'432'
 b'com.ubuntu.update-manager' b'window-width' b'564'
InstallationDate: Installed on 2013-01-30 (9 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Alpha i386 (20130129)
InterpreterPath: /usr/bin/python3.3
MarkForUpload: True
PackageArchitecture: all
ProcCmdline: /usr/bin/python3 /usr/bin/update-manager
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
PythonArgs: ['/usr/bin/update-manager']
SourcePackage: update-manager
Title: update-manager crashed with UnboundLocalError in show_diff(): local variable 'line_number' referenced before assignment
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo

[Apport retracing service (apport)]

Thank you for taking the time to report this crash and helping to make this software better. This particular crash has already been reported and is a duplicate of bug #1120322, so is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Please continue to report any other bugs you may find.