title: Displaying existing view controller in swift programmatically
body: <p>I have several view controllers embedded within a single navigation controller. 
  How would I display an earlier vc from a vc further down.
  This is what I have tried:</p>
  
  <pre><code>let vc = self.storyboard?.instantiateViewControllerWithIdentifier("MenuViewController") as! MenuViewController
          self.presentViewController(vc, animated: true, completion: nil)
  </code></pre>
  
  <p>This doesn't work and returns the following error:</p>
  
  <pre><code>fatal error: unexpectedly found nil while unwrapping an Optional value
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Y3eE6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Y3eE6.png" alt="enter image description here"></a></p>