Fix crash in release-build.

Fixed a crash bug in release-build. 
(`x as String` didn't work properly when `x` is `Optional<Optional<T>>`)

But there still seems to have Swift compiler bug when release-building...

1st hint from: https://github.com/ReactKit/ReactKit/commit/36fa3ce05e590ec405db86b37977b5712bba3769#commitcomment-8973134,