There is a crash when a device is registered in FCM.

1. The user setup to receive push notifications.
2. The user doesn't want to receive push notifications (unregister him device from FCM)
3. And then the user tries to register again him device in FCM

the pendingTopics stack has some values in null.

Author: Humberto Castaneda

Plugin.FirebasePushNotification/FirebasePushNotificationManager.ios.cs

@@ -551,16 +551,17 @@ public void DidReceiveRegistrationToken(Messaging messaging, string fcmToken)
                 while (pendingTopics.Count > 0)
                 {
                     var pTopic = pendingTopics.Dequeue();
-
-                    if (pTopic.Item2)
+                    if (pTopic != null)
                     {
-                        CrossFirebasePushNotification.Current.Subscribe(pTopic.Item1);
-                    }
-                    else
-                    {
-                        CrossFirebasePushNotification.Current.Unsubscribe(pTopic.Item1);
+                        if (pTopic.Item2)
+                        {
+                            CrossFirebasePushNotification.Current.Subscribe(pTopic.Item1);
+                        }
+                        else
+                        {
+                            CrossFirebasePushNotification.Current.Unsubscribe(pTopic.Item1);
+                        }
                     }
-
                 }
             }