Software updates window not shown

When updated packages are available, the unity bar shows the "Software updates" icon, but clicking on it no window is shown. The only way to do the updates, is to right-click on the icon and select "Install available updates" in the context menu.

Using Alt+Tab to select the "Software updates" window does not show the window.

The unity bar always shows the "Software updates" window as focused (blu arrow on the left of the icon and red border), no matter what is the currently focused window.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: update-notifier 0.134
ProcVersionSignature: Ubuntu 3.8.0-27.40-generic 3.8.13.4
Uname: Linux 3.8.0-27-generic i686
ApportVersion: 2.9.2-0ubuntu8.3
Architecture: i386
Date: Sat Aug 31 07:12:06 2013
ExecutablePath: /usr/bin/update-notifier
MarkForUpload: True
ProcEnviron:
 PATH=(custom, user)
 XDG_RUNTIME_DIR=<set>
 LANG=it_IT.UTF-8
 SHELL=/bin/bash
SourcePackage: update-notifier
UpgradeStatus: No upgrade log present (probably fresh install)