Add defensive checks before calling isFocused

[bausmeier]

When making use of useRef for refs forwarded to a custom functional
component there is potential for the isFocused function not to be
defined. Adding some extra checks to make sure that it is a function
before calling it prevents an error from being thrown.

[richardgirges]

It would be great to get this one merged! We're running into scenarios where the ref may be null.

[terryatgithub]

yes, come across the same issue, pls help solve it asap, thanks alot

TypeError: Cannot read property 'isFocused' of null
at KeyboardAwareBase.js:84
at Array.some ()
at KeyboardAwareScrollView._scrollToFocusedTextInput (KeyboardAwareBase.js:83)
at KeyboardAwareScrollView._onKeyboardWillShow (KeyboardAwareBase.js:98)
at RCTDeviceEventEmitter.emit (EventEmitter.js:189)
at MessageQueue.__callFunction (MessageQueue.js:416)
at MessageQueue.js:109
at MessageQueue.__guard (MessageQueue.js:364)
at MessageQueue.callFunctionReturnFlushedQueue (MessageQueue.js:108)
at debuggerWorker.aca173c4.js:4