Fix NPE during search

Summary:
`getChilAt` can return `null` (see also line 671), probably when the UI changes during the inspection process. Added a null check that should protect against the failure reported in P501013941

Changelog: Fixed NPE during search in layout plugin (Android)

Reviewed By: lblasa

Differential Revision: D36369508

fbshipit-source-id: 1f066d1a73bdfe875d66f7200ded0d0776af8321

android/src/main/java/com/facebook/flipper/plugins/inspector/InspectorFlipperPlugin.java

@@ -624,6 +624,9 @@ public class InspectorFlipperPlugin implements FlipperPlugin {
   }
 
   public SearchResultNode searchTree(String query, Object obj, boolean axEnabled) throws Exception {
+    if (obj == null) {
+      return null;
+    }
     final NodeDescriptor descriptor = descriptorForObject(obj);
     List<SearchResultNode> childTrees = null;
     boolean isMatch = descriptor.matches(query, obj);