Thanks for the documentation and the code you are posted here.
Like Milkyway reported, i had the same issue with “fireEvent” of undefined, due to invalid context object in the publish() method in Ext.ux.mixin.Mediator class. I have corrected it and sent you a pull request (https://github.com/12ftguru/extjs-ux-mediator/pull/2).
Your solution works now in my project. Many thanks! :)