Add defensive check before registerInputHandler in view component.

[terryatgithub]

• Requirement:
  add defensive check before registerInputHandler in view component.

• Issue:
  Since hyperview uses KeyboardAwareScrollView in 'react-native-keyboard-aware-scrollview', if inputRefs contain null, it would cause some random problems in some certain scenarioes as below:

1. 'Add defensive checks before calling isFocused #44',
2. iOS: when text-field is focused, throw exception: TypeError null is not an object (evaluating 'o.isFocused') #232

• For references:
  
  Caveats with callback refs

[cengit]

great job!
since react-native-keyboard-aware-scrollview is not so active now, this will help to fix my issue.
it would be great to include this PR and publish new version, thanks! @adamstep

[adamstep]

Thanks @terryatgithub !

[terryatgithub]

Thanks @terryatgithub !

It's ok. Thanks for your guys's great job, we are benifiting from the features of hyperview.