Show Game Centre Leaderboard not working in swift

I am coding in swift and I am getting the following error message from my submit button. The leaderboard does not show up in the simulator, just gives me the error message below

<GKGameCenterViewController: 0x7a8d0800> on     <Chinese_Quiz.OpeningViewController: 0x78688aa0> whose view is not in the  window hierarchy!

Below is all the game centre code from my app.

  override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    Randomize()
    Hide()
    authenticateLocalPlayer()

}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}
@IBAction func Submit(sender: AnyObject) {
    saveHighscore(Score)
    showLeaderboard()

}

func authenticateLocalPlayer(){

    var localPlayer = GKLocalPlayer.localPlayer()

    localPlayer.authenticateHandler = {(viewController, error) -> Void in

        if (viewController != nil) {
            self.presentViewController(viewController, animated: true, completion: nil)
        }

        else {
            println((GKLocalPlayer.localPlayer().authenticated))
        }
    }

}

func gameCenterViewControllerDidFinish(gameCenterViewController: GKGameCenterViewController!)
{
    gameCenterViewController.dismissViewControllerAnimated(true, completion: nil)

}

func showLeaderboard() {
    var vc = self.view?.window?.rootViewController
    var gc = GKGameCenterViewController()
    gc.gameCenterDelegate = self
    vc?.presentViewController(gc, animated: true, completion: nil)
}



    func saveHighscore(score:Int) {

    //check if user is signed in
    if GKLocalPlayer.localPlayer().authenticated {

        var scoreReporter = GKScore(leaderboardIdentifier: "ChineseWeather") //leaderboard id here

        scoreReporter.value = Int64(Score) //score variable here (same as above)

        var scoreArray: [GKScore] = [scoreReporter]

        GKScore.reportScores(scoreArray, withCompletionHandler: {(error : NSError!) -> Void in
            if error != nil {
                println("error")
            }
        })

    }

}