Fix exception in scrollResponderScrollNativeHandleToKeyboard when ref…

… is null

Summary:
We are seeing these errors in prod:

```
TypeError: Cannot read property '_nativeTag' of null

at
ReactNativeFiberHostComponent.prototype.measureLayout(ReactNativeRenderer-prod.fb.js:1594)
ScrollResponderMixin.scrollResponderScrollNativeHandleToKeyboard(ScrollResponder.js:557)
```

This error is coming from these lines: https://github.com/facebook/react-native/blob/69c38e5a639f34620038ae5724426c92c355e509/Libraries/Components/ScrollResponder.js#L563-L567

Either `nodeHandle` is null or `this.getInnerViewRef()`. If `nodeHandle` was null, we'd get an error that we can't call `measureLayout` on null. So `this.getInnerViewRef()` must be null.

In the React Native Renderer this error of `_nativeTag of null` is coming from this line: https://github.com/facebook/react/blob/db8afe4f6318dba422177a2054204ef089570ad8/packages/react-native-renderer/src/ReactNativeFiberHostComponent.js#L84

Which means indeed `this.getInnerViewRef()` is null.

So adding a null check here which is what we do at all the other product callsites of `measureLayout`. Flow should have caught this, but because ScrollView is one of the only components left using mixins (ScrollResponder), `this.getInnerViewRef` is typed as `any` instead of what it should be:

```
?React.ElementRef<Class<ReactNative.NativeComponent<mixed>>>
```

If `scrollResponder` was typed correctly then Flow would have caught this.

Changelog:
[Fixed] Exception in scrollResponderScrollNativeHandleToKeyboard when ref is null

Reviewed By: mmmulani

Differential Revision: D17717150

fbshipit-source-id: d7bc4c897ad259fb588e8100f37ccfb8a5d07874

Libraries/Components/ScrollResponder.js

@@ -570,8 +570,14 @@ const ScrollResponderMixin = {
         this.scrollResponderInputMeasureAndScrollToKeyboard,
       );
     } else {
+      const innerRef = this.getInnerViewRef();
+
+      if (innerRef == null) {
+        return;
+      }
+
       nodeHandle.measureLayout(
-        this.getInnerViewRef(),
+        innerRef,
         this.scrollResponderInputMeasureAndScrollToKeyboard,
         this.scrollResponderTextInputFocusError,
       );