title: I get a warning saying that the completion handler was never called When I simulate a Background fetch
body: <p>I followed all the steps in order to set up the <code>background fetch</code> but I'm suspecting that I made a mistake when writing the function <code>performFetchWithCompletionHandler</code>in the AppDelegate.</p>
  
  <p>Here is the warning that I get as soon as I simulate a <code>background fetch</code></p>
  
  <pre><code>Warning: Application delegate received call to -  application:
  performFetchWithCompletionHandler:but the completion handler was never called.
  </code></pre>
  
  <p>Here's my code :</p>
  
  <pre><code>func application(application: UIApplication, performFetchWithCompletionHandler completionHandler: (UIBackgroundFetchResult) -&gt; Void) {
      if let tabBarController = window?.rootViewController as? UITabBarController,
              viewControllers = tabBarController.viewControllers as [UIViewController]! {
        for viewController in viewControllers {
          if let notificationViewController = viewController as? NotificationsViewController {
           firstViewController.reloadData()
           completionHandler(.NewData)
           print("background fetch done")
        }
      }
    }
  }
  </code></pre>
  
  <p>How can I test if the <code>background-fetch</code>is working ? </p>