title: UIDocumentPicker navigation bar buttons are hidden at iOS 11
body: <p>I notice a problem in my UIDocumentPicker's navigation bar at iOS 11 only, the done, cancel, or edit buttons are invisible, and when the user touch it it appears i.e. The color at normal state is white, even when changing the <code>UINavigationBar.appearnce().tintColor</code>, The color only changed on touch.</p>
  
  <p><a href="https://i.stack.imgur.com/4bMrk.png" rel="noreferrer"><img src="https://i.stack.imgur.com/4bMrk.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/u0TQ7.png" rel="noreferrer"><img src="https://i.stack.imgur.com/u0TQ7.png" alt="enter image description here"></a></p>