(Swift SpriteKit) Only allow SKScene to present once

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.

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)

    }