Accessing timeline objects from a class, giving null object reference?

Question

All late comers this question in still active a answer is not yet reached, what you might see below is a irrelevent syntax error a nice member found for me

error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at Player()
    at Maintest_fla::MainTimeline/createPlayer()

When i'm attempted to add the instance name wall0x objects that are in the object with the instance name world, I find that I get a null object error. Also ignore the long list of variables, not relevant.

package 
{

    import flash.display.MovieClip;
    import flash.events.Event;
    import flash.events.MouseEvent;
    import flash.events.TimerEvent;
    import flash.filters.BlurFilter;
    import flash.utils.Timer;



    public class Player extends MovieClip
    {
        // player settings



        private var _rotateSpeedMax:Number = 20;
        public var _gravity:Number = .10;
        // projectile gun settings
        public var _bulletSpeed:Number = 4;
        public var _maxDistance:Number = 200;
        public var _reloadSpeed:Number = 250;//milliseconds
        public var _barrelLength:Number = 20;
        public var _bulletSpread:Number = 5;
        // gun stuff
        private var _isLoaded:Boolean = true;
        private var _isFiring:Boolean = false;
        private var _endX:Number;
        private var _endY:Number;
        private var _startX:Number;
        private var _startY:Number;
        private var _reloadTimer:Timer;
        private var _bullets:Array = [];

        // array that holds walls

        public var _solidObjects:Array = [];

        //
        private var _player:MovieClip;
        private var _dx:Number;
        private var _dy:Number;
        private var _pcos:Number;
        private var _psin:Number;
        public var _trueRotation:Number;



        public function Player()
        {

            // constructor code   //Right hereVVVthe instance name is wall0x and it's in the object world on the stage.                            
                _solidObjects = [MovieClip(root).world.wall01,MovieClip(root).world.wall02,MovieClip(root).world.wall03,MovieClip(root).world.wall04];


            /*addEventListener(Event.ENTER_FRAME, enterFrameHandler);
            addEventListener(MouseEvent.MOUSE_DOWN, onMouseDownHandler);
            addEventListener(MouseEvent.MOUSE_UP, onMouseUpHandler);*/
           }

        }
}

Code I'm using in frame 2 create the player and then continuously set it's chords to another objects.

stage.addEventListener(Event.ENTER_FRAME, createPlayer);
function createPlayer(e:Event):void
        {


            // attach player movieclip from library

            // position player in center
            if (character!=null&&_player!=null)
            {

                _player.x = character.x + 5;
                _player.y = character.y + 5;
            }
            else if (_player ==null && world.wall01 != null)
            {
            var _player:Player;


                _player = new Player();

                // add to display list
                stage.addChild(_player);
            }
        }

Answer 1

First: You have a syntax error in these two lines:

_player.x = MovieClip.(root).character.x + 5;
_player.y = MovieClip.(root).character.y + 5;

There should not be a period after MovieClip, so it should look like this:

_player.x = MovieClip(root).character.x + 5;
_player.y = MovieClip(root).character.y + 5;

Second: You are always creating a new Player every frame. In your createPlayer method, you have the following conditional:

if(character != null && _player != null)  //_player is not a defined in this scope, so it will either throw an error, or always return null/undefined

You do not have a _player var defined in the scope of that frame or the scope of the createPlayer method, you have defined it inside the scope of the else statement (which makes only available in the else statement)

Move the var _player:Player to the top of your timeline code with the other frame scoped vars.

Third: you are trying to access root in your Player constructor, the problem with this, is that when the contructor runs, your Player is not yet on the display tree, so root is null until you've added player to the stage.

example:

_player = new Player(); //this will run your contructor, but root will be null
stage.addChild(_player); //after this, your Player class will now have access to root/stage/parent object

Change your Player class so it listens for being ADDED_TO_STAGE before trying access root.

    public function Player()
    {
        this.addEventListener(Event.ADDED_TO_STAGE, init);
        // constructor code

    }

    private function init(e:Event):void {
        this.removeEventListener(Event.ADDED_TO_STAGE, init);
        _solidObjects = [MovieClip(root).world.wall01,MovieClip(root).world.wall02,MovieClip(root).world.wall03,MovieClip(root).world.wall04];
        addEventListener(Event.ENTER_FRAME, enterFrameHandler);
        addEventListener(MouseEvent.MOUSE_DOWN, onMouseDownHandler);
        addEventListener(MouseEvent.MOUSE_UP, onMouseUpHandler);
    }