title: Cannot receive push notification on iOS from Firebase 3.2.0 topics
body: <p>I followed the tutorial by google on <a href="https://firebase.google.com/docs/notifications/ios/console-topics#receive_and_handle_topic_messages" rel="noreferrer">https://firebase.google.com/docs/notifications/ios/console-topics#receive_and_handle_topic_messages</a> to subscribe to a Firebase topic on my iOS app. </p>
  
  <pre><code>func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject : AnyObject]?) -&gt; Bool {
  
      FIRMessaging.messaging().subscribeToTopic("/topics/Notifications")
  
      let homeViewController = UINavigationController(rootViewController: HomeViewController())
  
      UINavigationBar.appearance().translucent = false
      window = UIWindow(frame: UIScreen.mainScreen().bounds)
      window?.rootViewController = homeViewController
  
      window?.makeKeyAndVisible()
      return true
  }
  </code></pre>
  
  <p>However, when I send a topic push notification out from the Firebase console. I could not receive any push notifications. But when I send out push notification to user segment from the console, the push is working perfectly. When I check the Xcode console, I am seeing this FIRMessaging error.</p>
  
  <pre><code>2016-05-31 11:11:47.893: &lt;FIRMessaging/WARNING&gt; Cannot subscribe to topic: /topics/Notifications with token: (null) 
  </code></pre>
  
  <p>I've tried to search for this error but have no luck finding anything. I am not sure if this is the problem that is causing my app to not receive any push from topics. </p>
  
  <p>Does anyone have this issue and know how to solve it?</p>