ios - Show Game Centre Leaderboard not working in swift -

i coding in swift , getting following error message submit button. leaderboard not show in simulator, gives me error message below

<gkgamecenterviewcontroller: 0x7a8d0800> on     <chinese_quiz.openingviewcontroller: 0x78688aa0> view not in  window hierarchy! 

below game centre code app.

  override func viewdidload() {     super.viewdidload()     // additional setup after loading view, typically nib.     randomize()     hide()     authenticatelocalplayer()  }  override func didreceivememorywarning() {     super.didreceivememorywarning()     // dispose of resources can 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 signed in     if gklocalplayer.localplayer().authenticated {          var scorereporter = gkscore(leaderboardidentifier: "chineseweather") //leaderboard id here          scorereporter.value = int64(score) //score variable here (same above)          var scorearray: [gkscore] = [scorereporter]          gkscore.reportscores(scorearray, withcompletionhandler: {(error : nserror!) -> void in             if error != nil {                 println("error")             }         })      }  } 

solved turning this:

func showleaderboard() { var vc = self.view?.window?.rootviewcontroller var gc = gkgamecenterviewcontroller() gc.gamecenterdelegate = self vc?.presentviewcontroller(gc, animated: true, completion: nil) } 

into this:

func showleaderboard() {     var vc = self     var gc = gkgamecenterviewcontroller()     var gamecenterdelegate: gkgamecentercontrollerdelegate!     // above text replaced gc.gamecenterdelegate = self     vc.presentviewcontroller(gc, animated: true, completion: nil) } 

i don't know why, worked.