Check if result from native UIManager is null before accessing view config

Summary:
As titled - attempt to mitigate a JS error on iOS while we work on getting JS view configs. It seems like this happens when the bridge is invalid.

Changelog: [Fixed][iOS] Don't throw an error if the UIManager returns null when fetching a view config

Reviewed By: kacieb

Differential Revision: D25247203

fbshipit-source-id: e2003f99b818f9657c60ff95b266be74fe18a94b

Libraries/ReactNative/PaperUIManager.js

@@ -71,7 +71,7 @@ const UIManagerJS = {
     ) {
       const result = NativeUIManager.lazilyLoadView(viewManagerName);
       triedLoadingConfig.add(viewManagerName);
-      if (result.viewConfig) {
+      if (result != null && result.viewConfig != null) {
         getConstants()[viewManagerName] = result.viewConfig;
         lazifyViewManagerConfig(viewManagerName);
       }