Crash - libswiftCore.dylib`_swift_abortRetainUnowned:

Hi @shingt ,
Why this app crashes on the runtime.
Build is successful.

![screen shot 2017-08-20 at 2 06 56 pm](https://user-images.githubusercontent.com/1847035/29492545-31e45b5a-85b1-11e7-9bb2-50c871cbd1c3.png)


![screen shot 2017-08-20 at 2 07 05 pm](https://user-images.githubusercontent.com/1847035/29492547-3aa8d194-85b1-11e7-9e37-2fb1a0b39a9a.png)
