title: flip card causes fatal error in swift
body: <p>I am following <a href="http://www.codingricky.com/flipping-cards-with-swift-and-uikit/" rel="nofollow noreferrer">this card flipping tutorial</a> and instead of creating the <code>UIView</code> and the <code>UIImageViews</code> programmatically I have created these in my storyboard.<br>
  When I click the card, the animation kicks in and turns over to the front image, but when I click again, this creates a <code>fatal error</code>:</p>
  
  <blockquote>
    <p>unexpectedly found nil while unwrapping an Optional value.</p>
  </blockquote>
  
  <p>I can not figure out why this error occurs.</p>
  
  <p>Here is the code which is the transformation of the above mentioned tutorial:</p>
  
  <pre><code>class FirstViewController: UIViewController {
  
  @IBOutlet weak var flashCardView: UIView!
  @IBOutlet weak var backImage: UIImageView!
  @IBOutlet weak var frontImage: UIImageView!
  
  
  var showingBack = true
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          let singleTap = UITapGestureRecognizer(target: self, action: Selector("tapped"))
          singleTap.numberOfTapsRequired = 1
  
          flashCardView.addGestureRecognizer(singleTap)
          flashCardView.addSubview(backImage)
          view.addSubview(flashCardView)
      }
  
      func tapped() {
          if (showingBack) {
              UIView.transitionFromView(backImage, toView: frontImage, duration: 1, options: UIViewAnimationOptions.TransitionFlipFromRight, completion: nil)
              showingBack = false
          } else {
              UIView.transitionFromView(frontImage, toView: backImage, duration: 1, options: UIViewAnimationOptions.TransitionFlipFromLeft, completion: nil)  
              //here I get the error when flipping the card back to the back image
              showingBack = true
          }
      }
  } 
  </code></pre>