Transitioning to viewController from SKScene found nil when unwrapping optional value

so i found some posts about transitioning from Skscene to uiviewcontroller and I got it to work. This segue and unwind segue is called everytime the user win the level of my game or loses.

this works for level1 but as soon as I win level2 I get

 fatal error: unexpectedly found nil while unwrapping an Optional value

on the line where I call the game over function below

in my game scene I have :

class GameScene: SKScene, SKPhysicsContactDelegate {

//next level / try again segue
var viewController : GameViewController!

in the GameViewController i initialize this property

    var currentLevel: Int!


override func viewDidLoad() {
    super.viewDidLoad()
    currentLevel = gameScene.currentLevel
    if let scene = GameScene.level(currentLevel) {
        // Configure the view.
        let skView = self.view 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)

        //initialize VC
        scene.viewController = self
    }

 func gameOver() {
    performSegueWithIdentifier("gameOver", sender: nil)
}

@IBAction func perpareForUnwind(unwindSegue: UIStoryboardSegue) {

}

and finally i call gameOver from my win() function in gameScene

    func newGame() {
    view!.presentScene(GameScene.level(currentLevel))
}

func win() {
    if (currentLevel < 3) {
        currentLevel++
        //present win view - with option to restart, next level, or home
    }
    println(currentLevel)
    runAction(SKAction.sequence([SKAction.waitForDuration(2),
        SKAction.runBlock(newGame)]))
    self.viewController.gameOver() // this is the error!
}

So this works from level1 to level2 but wont work from level2 to level3