I'm getting a slightly different Crash to Desktop bug now.
So I'm in the Behaviour editor of an object (I accessed it from within my "Engine" frame). I do a Find all search for something that appears in this object's behaviours. I can use the mouse/keyboard to select search results in the Find Toolbar that belong to the "Engine" frame (eg. the blue one below). But when I select one that's from a different frame (eg. the "Foyer" frame), Fusion crashes.