twitters app url includes /None/ which 404's

[Dave Morley]

Bug Description:
Goto an app click on the twitter post option. Wait for the url to show up in twitter note the url back to apps.ubuntu.com contains a None this shouldn't be the case and causes a 404 to happen.

This only happen in the general view if you click on a specific version None is replace by the Distro codename target instead.

UAD Version:
Django 1.3
Python 2.6.5 (r265:79063, Apr 16 2010, 14:15:55) [GCC 4.4.3]
preflight 0.1

OS:
64bit Oneiric

Browser:
FF 11.0

Problem URL:
https://apps.ubuntu.com/cat/applications/geany/
https://apps.ubuntu.com/cat/applications/precise/geany/

Steps To Reproduce:
1. goto the the first link above
2. click on the twitter send message
3. check the url in twitter
4. goto the second link above
5. click on the twitter send message
6. check out the url in twitter

Expected Result:
Neither of the links above should 404. The None should not be present.