Replay problem

When i play normally, i didnt encounter any problem...but when i pause and replay the animation, it show Cannot access a property or method of a null object reference.
at line as:80. What is the reason ?how can i solve it?

package
{
          import flash.display.MovieClip;
          import flash.events.KeyboardEvent;
          import flash.ui.Keyboard;
          import flash.events.Event;
          import flash.events.MouseEvent;
          import flash.ui.Mouse;
          import flash.utils.Timer;
          import flash.events.TimerEvent;
  
  
          public class Assignment extends MovieClip
          {
                    var vx:int;
                    var vy:int;
                    var collisionHasOccurred:Boolean;
                    var score:uint;
  
                    public function Assignment()
                    {
                              init();
  
                    }
                    function init():void
                    {
                              //Initialize variable
                              vx=0;
                              vy=0;
                              collisionHasOccurred=false;
                              stone.stop();
                              score=0;
                              optionPage.visible=false;
  
                    //Add event listeners
                    stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);
                    stage.addEventListener(KeyboardEvent.KEY_UP, onKeyUp);
                    addEventListener(Event.ENTER_FRAME, onEnterFrame);
                    }
  
                    function onKeyDown(event:KeyboardEvent):void
                    {
                              if(event.keyCode==Keyboard.LEFT)
                              {
                                        vx=-5;
                              }
                              else if (event.keyCode==Keyboard.RIGHT)
                              {
                                        vx=5;
                              }
                              else if (event.keyCode==Keyboard.UP)
                              {
                                        vy=-5;
  
                              }
                              else if (event.keyCode==Keyboard.DOWN)
                              {
                                        vy=5;
                              }
  
                    }
                    function onKeyUp(event:KeyboardEvent):void
                    {
                              if (event.keyCode==Keyboard.LEFT || event.keyCode==Keyboard.RIGHT)
                              {
                                        vx=0;
                              }
                              else if (event.keyCode==Keyboard.DOWN || event.keyCode==Keyboard.UP)
                              {
                              vy=0;
                              }
                    }
                    function onEnterFrame(event:Event):void
                    {
                              //Move the player
                              player.x+=vx;
                              player.y+=vy;
  
                              //collision detection
                              if (stone.hitTestObject(player))<-----here is line 80
                    { 
                              player.gotoAndStop(3);
                              health.meter.width-=2;
                              if (! collisionHasOccurred)
                              {
                                        score++;
                                        messageDisplay.text=String(score);
                                        collisionHasOccurred=true;
                              }
          }
                    else
                    {
                              player.gotoAndStop(1);
                              collisionHasOccurred=false;
                    }
  
                    if (health.meter.width <=1)
                    {
                              New.text="Game Over!";
                              stop();
                              fl_CountDownTimerInstance_3.stop();
  
                    }
                    if (player.hitTestObject(wall))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallA))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallB))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallC))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallD))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallE))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallF))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallG))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
                    if (player.hitTestObject(wallH))
                    {
                              player.x-=vx;
                              player.y-=vy;
                    }
  
  
                              }
          }
}