Make sure keyboardWill(Show|Hide)Event exists (#79)

It can be undefined causing app crashes

Author: pietropizzi

lib/KeyboardAwareMixin.js

@@ -93,8 +93,8 @@ const KeyboardAwareMixin = {
   },
 
   componentWillUnmount: function () {
-    this.keyboardWillShowEvent.remove()
-    this.keyboardWillHideEvent.remove()
+    this.keyboardWillShowEvent && this.keyboardWillShowEvent.remove()
+    this.keyboardWillHideEvent && this.keyboardWillHideEvent.remove()
   },
 
   scrollToPosition: function (x: number, y: number, animated: bool = false) {