Guard against result of getUIManager(...) being null

Summary:
`getUIManagerX(...)` can return null now. Guard in a few places that use it or add comments.

Changelog: [Internal]

Reviewed By: alexeylang

Differential Revision: D18351197

fbshipit-source-id: f077835468a75d1af24cfb4210989ba875ff9086

ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java

@@ -1080,6 +1080,10 @@ public class ReactInstanceManager {
   private void attachRootViewToInstance(final ReactRoot reactRoot) {
     Log.d(ReactConstants.TAG, "ReactInstanceManager.attachRootViewToInstance()");
     Systrace.beginSection(TRACE_TAG_REACT_JAVA_BRIDGE, "attachRootViewToInstance");
+
+    // UIManager is technically Nullable here, but if we can't get a UIManager
+    // at this point, something has probably gone horribly wrong so it's probably best
+    // to throw a NullPointerException.
     UIManager uiManager =
         UIManagerHelper.getUIManager(mCurrentReactContext, reactRoot.getUIManagerType());