Adding an element to a newly-added List fails in a fresh window

(Bug, Closed -> Fixed, Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
User Interface
Summary: Create a Model. Add a List of Text Property. Refresh the whole window, from the browser. Add an element to that list. It fails, and if you reload again, the element isn't there.
The issue is a consequence of our using "mainThing" as a side-effecting crutch. The Model Designer page isn't setting that, so if you reload the editor from scratch, without going through the Model's page, it is None, and things go splat.