React-native-camera: Camera Permission popup

[JJMoon]

Steps to reproduce

1. Get the Camera permission on startup.
2. Turn off camera permission in device setting
3. Return to the app and go to camera screen.

Expected behaviour

Show permission popup

Actual behaviour

Show blank screen with 'camera not authorized'
(It crashed at v 0.12.0)

Environment

• Node.js version: 9.3.0
• React Native version: 0.50
• React Native platform + platform version: iOS 9.0
  I didn't test on Android device.

react-native-camera

Version: npm version or "master"

It is not critical, but (I hope) should be fixed.

[sibelius]

You can only show permission popup once, if user denied, he needs to go to settings and reenable