Fix for: 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderSet...

... initWithObjects:count:]: attempt to insert nil object from objects[0]'

The code above this appears to be doing nil checking of instance, whereas this code doesn't, and crashes when instance is nil. I sent it out to our beta testers and already had 22 crashes in an hour so it's definitely a real issue ;)

I'm not familiar enough with FCModel to know if you need to move some of the code above into a common method and use it in both cases or not, but this doesn't appear to be the case?
These may not need to be mutable?