[BUGFIX relationships] fix proxy isDestroying error

[code0100fun]

This fixes an error with async belongsTo relationships throwing an error like:

Cannot read property 'isDestroying' of null

This happens when the relationship fetch fails the first time and is then accessed again later.

[runspired]

I can't figure out looking at this why the jquery scenario would behave differently, you likely should try running that locally so you can poke at the failure