I'm not sure how well supported this situation is supposed to be,
but the following example from @Demeter Dzadik (Mets) would fail when using the
custom property edit operator, because it would try to retrieve
UI data for a group property:
C.object['abuse'] = {'parent' : ['child1', 'child2', 'why', 'would', 'you', 'do', 'this']}

I'm not really thrilled with this approach, but it does seem to fix
the issue. I would much rather see this operator changed to that
it has a type drop-down at the top of the popup, and then properly
exposed properties depending on the type. Then if the custom
property was one of these weird types it could avoid dealing with
the UI data completely. I think I'm confident enough in this area
to work on that now, but this case should probably be fixed first.