title: Could not cast value of type 'nsnull' to 'nsstring'
body: <p>I'm trying to retrieve images from firebase when I start the simulated it crash and I get this message " <strong>Could not cast value of type 'NSNull' (0x10eaf2378) to 'NSString' (0x10eee2b20).</strong> "
  this is my code</p>
  
  <p>class ViewController: UIViewController {</p>
  
  <pre><code>@IBOutlet weak var gameName: UILabel!
  @IBOutlet weak var videoImage: UIImageView!
  @IBOutlet weak var uploadImage: UIButton!
  
  
  var ref = Firebase(url:"https://xxx.firebaseio.com/images/")
  var imageRef = Firebase(url:"https://xxx.firebaseio.com/images/image/string")
  var base64String: NSString!
  
  override func viewDidLoad() {
      super.viewDidLoad()
      // Do any additional setup after loading the view, typically from a nib.
  
  
      imageRef.observeEventType(.Value, withBlock: { (snapshot) -&gt; Void in
  
          self.base64String = snapshot.value as? String
          let decodeData = NSData(base64EncodedString: snapshot.value as! String, options: NSDataBase64DecodingOptions())
          let decodeImage = UIImage(data: decodeData!)
          self.videoImage.image = decodeImage
          print(self.base64String)
          }) { (error) -&gt; Void in
              print(error.description)
      }
  
  }
  </code></pre>