uncaught exception: Permission denied to get property Window.App

I got this after upgrading my MovableType installation every time I tried to add/edit a template or entry. The problem was with the domain I was using not being the same as the one set in mt/mt-config.cgi

For example, in mt/mt-config.cgi I had

CGIPath                 http://robbiebow.co.uk/mt/

But I was accessing the control panel at

http://www.robbiebow.co.uk/mt/

Changing one to be the same as the other and the problem was solved.