title: Focus disappearing when opening a UIAlertController on tvOS 10 GM
body: <p>I want to display a <code>UIAlertController</code> on top of a <code>UIViewController</code> with a <code>UICollectionView</code> inside. The collection view needs to focus on startup, so I overrode the <code>preferredFocusableView</code> variable as follows:</p>
  
  <pre><code>override var preferredFocusedView: UIView? {
      return self.collectionView
  }
  </code></pre>
  
  <p>With tvOS 9 all worked fine: the alert controller opened properly and I was able to choose one of the <code>UIAlertAction</code>s displayed.</p>
  
  <p>On tvOS 10 Golden Master, after opening the alert controller and scrolling to another action, the focus disappears from the screen and I'm unable to scroll to other actions or tapping on the Menu button of the Siri Remote. The app remains stuck in the alert controller and I can hear the scrolling sound when I try to scroll to other actions but nothing happens on screen. I have to force quit the app and reopen it.</p>
  
  <p><a href="https://i.stack.imgur.com/3S2bF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3S2bF.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/ClU7g.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ClU7g.png" alt="enter image description here"></a></p>
  
  <p>This is the code of the app. I tried to set the <code>preferredFocusableView</code> to <code>alertController.preferredFocusedView</code> or by removing the focus methods of the collection view but with no results.</p>
  
  <pre><code>var alertController : UIAlertController?
  
  func showAlert() {
  
      alertController = UIAlertController(title:"Example title", message: "Example description", preferredStyle: .Alert)
  
      let action1 = UIAlertAction(title: "Option 1", style: .Default) { (action : UIAlertAction) -&gt; Void in
          //call to another method
      }
  
      // action2, action3, action4...
  
      let action5 = UIAlertAction(title: "Option 5", style: .Default) { (action : UIAlertAction) -&gt; Void in
          //call to another method
      }
  
      let actionDismiss = UIAlertAction(title: "Dismiss", style: .Destructive) { (action : UIAlertAction) -&gt; Void in
          self.alertController!.dismissViewControllerAnimated(true, completion: nil)
      }
  
      alertController!.addAction(action1)
      alertController!.addAction(action2)
      alertController!.addAction(action3)
      alertController!.addAction(action4)
      alertController!.addAction(action5)
      alertController!.addAction(actionDismiss)
  
      alertController!.preferredAction = action1
  
      self.presentViewController(alertController!, animated: true, completion: nil)
  }
  
  override var preferredFocusedView: UIView? {
      if self.alertController != nil {
          return self.alertController!.preferredFocusedView
      } else {
          return self.collectionView
      }
  }
  </code></pre>