Fix NullPointerException when reporting mounts for mount hooks (#39022)

Summary:
Pull Request resolved: #39022

Fixes a possible NullPointerException thrown when trying to access the binding after the instance has been destroyed to report mounts.

I added a check for the mount item just in case 😅

Changelog: [internal]

Reviewed By: lenaic

Differential Revision: D48355738

fbshipit-source-id: 401d2e0a52b0764ed89498ecc0176d160226e509

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -1227,20 +1227,21 @@ public void didMountItems(@Nullable List<MountItem> mountItems) {
               public void run() {
                 mMountNotificationScheduled.set(false);
 
-                if (mountItems == null) {
+                final @Nullable Binding binding = mBinding;
+                if (mountItems == null || binding == null) {
                   return;
                 }
 
                 // Collect surface IDs for all the mount items
                 List<Integer> surfaceIds = new ArrayList();
                 for (MountItem mountItem : mountItems) {
-                  if (!surfaceIds.contains(mountItem.getSurfaceId())) {
+                  if (mountItem != null && !surfaceIds.contains(mountItem.getSurfaceId())) {
                     surfaceIds.add(mountItem.getSurfaceId());
                   }
                 }
 
                 for (int surfaceId : surfaceIds) {
-                  mBinding.reportMount(surfaceId);
+                  binding.reportMount(surfaceId);
                 }
               }
             });