While an existing transition or presentation is occurring; the navigation stack will not be updated
<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>