Fix crash that occurs when FXFormFieldKey is absent

`NSManagedObject` overrides `respondsToSelector:` in such a way that it
crashes when `nil` is passed in.

This commit fixes the two spots where respondsToSelector is called with
a possible nil value.

This fixes #42
Merge pull request #69 from bartvandendriessche/fix/NSManagedObject_now_handles_missing_FXFormFieldKey

Fix crash that occurs when FXFormFieldKey is absent