Summary: I noticed that sometimes the batched bridge would be valid before `[self enqueueApplicationScript:url:onComplete:]` but then become invalid in the completion callback. This diff checks `self.isValid` inside of the callback.
Closes https://github.com/facebook/react-native/pull/2016
Reviewed By: @trunkagent, @jspahrsummers
Differential Revision: D2443438
Pulled By: @tadeuzagallo