#1009: Cannot access a property or method of a null object reference

I have .fla file with 2 keyframes:

1 its screen where player plaing..

2 its screen where player see message like 'you are winner'

Also i separated code from .fla file.

So i can not clear why i see this #1009 error after i go to label 'p2'.

Can anyone explaine why its happend and how i can solve it?

attached image for help.

code:

package {

     import flash.events.*;
     import flash.display.*;
     import flash.text.TextField;

     public class GuessNumber extends MovieClip {
          var guessNumber:Number;
          var points:Number = 0;     
          var level:Number = 1;
          var lastGuessNumber:Number = 5;
          
          //constructor
          public function GuessNumber() {
               pointsView.text = "0";
               levelView.text = "1";
               addEventListener(Event.ADDED_TO_STAGE,startGuess);
          }
          
          private function startGuess(e:Event=null):void {
               guessBtn.useHandCursor=true;
               guessBtn.enabled=true;
               playAgainBtn.useHandCursor=true;
               playAgainBtn.enabled=false;
               levelNumbers.text="betwen 1 and " +  lastGuessNumber;
               yourGuessText.text = "";
               levelView.text = String(level);
               pointsView.text = String(points);
               
               guessNumber=Math.ceil(Math.random()*lastGuessNumber);
               trace(guessNumber);
               
               removeEventListener(Event.ADDED_TO_STAGE,startGuess);
               guessBtn.addEventListener(MouseEvent.CLICK,checkingYourGuess);
          }
          
          private function checkingYourGuess(e:MouseEvent):void {
               if (guessNumber==Number(yourGuessText.text)) {
                    outputText.text = "You are GUESS!!!";
                    points += 100;                    
                    winGame();
               } else if (guessNumber > Number(yourGuessText.text)) {
                    points -= 20;
                    pointsView.text = String(points);
                    outputText.text="need MORE, try again please";
               } else if (guessNumber < Number(yourGuessText.text)) {
                    points -= 20;
                    pointsView.text = String(points);
                    outputText.text="need LESS, try again please";
               }
          }
          
          private function winGame():void {
               pointsView.text = String(points);
               if (points >= 200)
               {
                    points = 0;
                    level += 1;
                    lastGuessNumber += 10;
               }
               
               if (points>=200 || level==4) 
               {
                    trace("you are winner!!!!!!!!!!!!!!!!!");
                    youAreWinner();
               }
               
               playAgainBtn.enabled=true;
               guessBtn.enabled=false;
               playAgainBtn.addEventListener(MouseEvent.CLICK, playAgain);
          }
          
          private function playAgain(event:MouseEvent):void {
               outputText.text = "";
               startGuess();
          }
          
          private function youAreWinner():void
          {
               gotoAndStop('p2');
          }
     }

}