title: How to pass (optional) completion handler closure to transitionFromViewController in Swift?
body: <p>In a ViewController in my app I call transitionFromViewController but always get the following error when passing in a closure to the completion: argument. </p>
  
  <pre><code>Type '() -&gt; Void' does not conform to protocol 'NilLiteralConvertible'
  </code></pre>
  
  <p>Here's the function call:</p>
  
  <pre><code>    self.transitionFromViewController(
          self.currentVC,
          toViewController: newController,
          duration: 0.2,
          options: UIViewAnimationOptions.TransitionCrossDissolve,
          nil,
          completion: { finished in
              fromViewController.removeFromParentViewController()
              toViewController.didMoveToParentViewController(containerViewController)
              toViewController.view.frame = containerViewController.view.bounds
      })
  </code></pre>
  
  <p>According to code completion the method signature is as follows:</p>
  
  <pre><code>transitionFromViewController(fromViewController: UIViewController, toViewController: UIViewController, duration: NSTimeInterval, options: UIViewAnimationOptions, animations: () -&gt; Void(), completion: ((Bool) -&gt; Void)?)
  </code></pre>