title: While an existing transition or presentation is occurring; the navigation stack will not be updated
body: <p>I have encountered this warning:</p>
  
  <blockquote>
    <p>pushViewController:animated: called on  while an existing transition or presentation is
    occurring; the navigation stack will not be updated.</p>
  </blockquote>
  
  <p>While trying to call <code>navigationController?.popViewControllerAnimated(false)</code> from <code>UIAlertController</code> completion block.</p>