title: Custom BackButton on NavigationController not appearing
body: <p>I'm trying to use a custom back button in <code>UINavigationController</code> but the image is not appearing.</p>
  
  <p><a href="https://i.stack.imgur.com/1AUn4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1AUn4.png" alt="screenshot"></a></p>
  
  <p>Here's the code:</p>
  
  <pre><code>self.navigationController?.navigationBar.setBackgroundImage(UIImage(), forBarMetrics: .Default)
          self.navigationController?.navigationBar.shadowImage = UIImage()
          self.navigationController?.navigationBar.translucent = true
          self.navigationController!.view.backgroundColor = UIColor.clearColor()
  
          self.navigationController?.navigationBar.backIndicatorImage = UIImage(named: "Back-50")
          self.navigationController?.navigationBar.backIndicatorTransitionMaskImage = UIImage(named: "Back-50")
          self.navigationItem.backBarButtonItem = UIBarButtonItem(title: "", style: UIBarButtonItemStyle.Plain, target: nil, action: nil)
  </code></pre>
  
  <p>I'm not sure what's going wrong with this code.</p>