work around inspector redbox due to Fiber

Summary: This temporarily fixes redbox when using Inspector before Fiber is fully ready.

Reviewed By: sebmarkbage

Differential Revision: D4593324

fbshipit-source-id: 287df97f5ecf30c15890f7f7bca4646421bb41c8

Libraries/Inspector/InspectorUtils.js

@@ -21,7 +21,7 @@ function traverseOwnerTreeUp(hierarchy, instance) {
 
 function findInstanceByNativeTag(nativeTag) {
   var instance = ReactNativeComponentTree.getInstanceFromNode(nativeTag);
-  if (typeof instance.tag === 'number') {
+  if (!instance || typeof instance.tag === 'number') {
     // TODO(sema): We've disabled the inspector when using Fiber. Fix #15953531
     return null;
   }