Game Center Authentication Handler View Controller is always zero

I am trying to set up Game Center in a quick game with a sprite pack. The App Delegate does the following:

func authenticateLocalPlayer(){
    let localPlayer: GKLocalPlayer = GKLocalPlayer.localPlayer()

    localPlayer.authenticateHandler = {(viewController : UIViewController!, error : NSError!) -> Void in
        //handle authentication
        if (viewController != nil){
            self.window?.rootViewController?.presentViewController(viewController, animated: true, completion: nil)
        }
        else{
            if (localPlayer.authenticated){
                gameCenterEnabled = true

                //Get the default leaderboard identifier.
                localPlayer.loadDefaultLeaderboardIdentifierWithCompletionHandler({ (leaderboardIdentifierr: String!, error: NSError!) -> Void in
                    if (error != nil){
                        NSLog("%@", [error.localizedDescription])
                    }
                    else{
                        leaderboardIdentifier = leaderboardIdentifierr
                    }
                })
            }
            else{
                gameCenterEnabled = false
            }

        }
    }
}

      

        
        
        
      

    

The problem I am facing is that localPlayer.autheniticateHandler always returns nil viewController even though my local player is not authenticated. Please let me know what I am doing wrong and how to fix it.