title: Swift: navigationController returning nil
body: <p>On a UIViewController instantiated by a UITabBarController, when I run the following code, </p>
  
  <pre><code>let destination = self.storyboard?.instantiateViewControllerWithIdentifier("test")
  self.navigationController!.pushViewController(destination!, animated: true)
  </code></pre>
  
  <p><code>navigationController</code> return nil. How can I get <code>navigationController</code> instance to push a view controller?</p>