Refactor calls to UIManagerHelper.getUIManager

Summary:
This diff refactors the usages of UIManagerHelper.getUIManager() to make sure we always consider null objects.
Some of the callsites were throwing a NullPointerExcetpion, now they throw a more explicit exception.

changelog: [internal]

Reviewed By: makovkastar

Differential Revision: D19383064

fbshipit-source-id: 1806a37528e80cab1c8fdff5eb631aaf47bde819

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

@@ -1104,12 +1104,16 @@ public class ReactInstanceManager {
     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.
+    @Nullable
     UIManager uiManager =
         UIManagerHelper.getUIManager(mCurrentReactContext, reactRoot.getUIManagerType());
 
+    // If we can't get a UIManager something has probably gone horribly wrong
+    if (uiManager == null) {
+      throw new IllegalStateException(
+          "Unable to attache a rootView to ReactInstance when UIManager is not properly initialized.");
+    }
+
     @Nullable Bundle initialProperties = reactRoot.getAppProperties();
 
     final int rootTag =