Fix NPE in FetchedAppGateKeepersManager

Summary: There is a chance that callback from callbacks.poll() can be null, and it will introduce NPE issue

Reviewed By: dreamolight

Differential Revision: D17186898

fbshipit-source-id: 3f11e924c3485bb5611d11e5b88ecf358e0022be

facebook-core/src/main/java/com/facebook/internal/FetchedAppGateKeepersManager.java

@@ -148,12 +148,15 @@ private static void pollCallbacks() {
 
         while (!callbacks.isEmpty()) {
             final Callback callback = callbacks.poll();
-            handler.post(new Runnable() {
-                @Override
-                public void run() {
-                    callback.onCompleted();
-                }
-            });
+            // callback can be null when function pollCallbacks is called in multiple threads
+            if (callback != null) {
+                handler.post(new Runnable() {
+                    @Override
+                    public void run() {
+                        callback.onCompleted();
+                    }
+                });
+            }
         }
     }