Null Object Reference

programming flash
1

I’m getting this error when I click the start button from the main menu.
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Player()[C:\Users\Brandon\Desktop\Player.as:21]
at Engine()[C:\Users\Brandon\Desktop\Engine.as:13]
at MainMenu/playGame()[C:\Users\Brandon\Desktop\MainMenu.as:19]


package {
    
    import flash.display.MovieClip;
    import flash.display.Stage;
    import flash.events.Event;
    import flash.display.SimpleButton;
    import flash.events.MouseEvent;
    
    public class MainMenu extends MovieClip {
        
        public function MainMenu()
        {
            startButton.addEventListener(MouseEvent.MOUSE_DOWN, playGame, false, 0, true);
        }
 
        private function playGame(event:MouseEvent) : void
        {
            var engine:Engine = new Engine(stage);
            gotoAndStop(2);
        }

    }
}



package 
{

import flash.events.*;
import flash.utils.Timer;
import flash.display.*;

    public class Engine extends MovieClip
    {
        private var stageRef:Stage;
        public static var enemyArray:Array = new Array();
        public var scoreHUD:ScoreHUD = new ScoreHUD(stageRef);
        var player:Player = new Player(stageRef);
        private var myTimer:Timer = new Timer(1000);
        var i:Number = 1;
        
        public function Engine(stageRef:Stage)
        {
            this.stageRef = stageRef;
            stageRef.addEventListener(Event.MOUSE_LEAVE,go);
            myTimer.addEventListener(TimerEvent.TIMER, timerFunction);
            myTimer.start();

            createEnemies();

            stageRef.addChild(player);
            player.x = stageRef.stageHeight / 2;
            player.y = stageRef.stageWidth / 2;
        }
........