It's caused by a known (now fixed) bug in wxwidgets. Follow the steps & and the issue should be resolved in no time.
First, run the following command (only if curl isn't already installed):
sudo apt-get install curl
(Everything that follows is mandatory) Then issue this command:
curl http://apt.wxwidgets.org/key.asc | sudo apt-key add -
Then go to System Settings > Software Sources > "Other Software" tab > click "Add..." and add the following one after the other (or edit /etc/apt/sources.list file):
deb http://apt.wxwidgets.org/ natty-wx main
deb-src http://apt.wxwidgets.org/ natty-wx main
Then run the following commands one after the other:
sudo apt-get update
sudo apt-get install python-wxgtk2.8 python-wxtools wx2.8-i18n
That's it! You don't even have to restart your PC.
[source: detailed instructions here]