Prevent the race condition between AppGetOrCreateInstance and AppReleaseReference

This potentially fix b/121161177 which crash at callback::Terminate().
Also do nullptr check in callback::Terminate() to make it a bit more durable.

The result of the race is that the developer or any C# module may get a FirebaseApp through FirebaseApp.DefaultInstance or FirebaseApp.GetInstance() which is pointing to a C++ App scheduled to be deleted and result in double deletion to the same C++ App.

PiperOrigin-RevId: 251735913

Author: chkuang-g

app/src/callback.cc

@@ -230,7 +230,9 @@ void Terminate(bool flush_all) {
   // the outstanding number of items in the queue.  In particular,
   // PollDispatcher() could be executing at this point since g_callback_mutex
   // isn't held by the ref count is > 0 for the duration of the function.
-  if (flush_all) ref_count += g_callback_dispatcher->FlushCallbacks();
+  if (flush_all && g_callback_dispatcher) {
+    ref_count += g_callback_dispatcher->FlushCallbacks();
+  }
   Terminate(ref_count);
 }