title: (Swift SpriteKit) Only allow SKScene to present once
body: <p>This code resides in the viewDidLoad function. The first time it is called it is fine. But then I present a subView on top of it from the same storyboard file and when 'let skView = gameView as! SKView' gets called again and throws a Fatal error: Found nil... How do I stop it from being called the other times? I can't think of any checks I could but in place.</p>
  
  <pre><code>if let scene = GameScene(fileNamed:"GameScene") {
          // Configure the view.
  
          let skView = gameView as! SKView
          skView.showsFPS = true
          skView.showsNodeCount = true
  
          /* Sprite Kit applies additional optimizations to improve rendering performance */
          skView.ignoresSiblingOrder = true
  
          /* Set the scale mode to scale to fit the window */
          scene.scaleMode = .AspectFill
  
          skView.presentScene(scene)
  
      }
  </code></pre>