TypeError: Error #1009 in Actionscript 3

I am developing a Jigsaw puzzle in Flash. I am developing a class for puzzle piece. The code of the PuzzlePiece class in given as follows.

package 
{
import flash.display.MovieClip;
import flash.events.MouseEvent;

public class PuzzlePiece extends MovieClip
{

    private var pieceX:Number;
    private var pieceY:Number;

    private var pieceXRandom:Number;
    private var pieceYRandom:Number;


    public function PuzzlePiece(pieceXRandom:Number,pieceYRandom:Number)
    {
        this.pieceXRandom = pieceXRandom;
        this.pieceYRandom = pieceYRandom;
        this.addEventListener(MouseEvent.MOUSE_DOWN,Drag);

        positionClips();
        this.gotoAndStop(2);

        this.holder_mc.width = this.holder_mc.height = 60;
        this.mask1_mc.width = t开发者_JAVA百科his.mask2_mc.width = 60;
        this.mask1_mc.height = this.mask2_mc.height = 60;

    }

    private function positionClips():void
    {
        this.x = pieceXRandom;
        this.y = pieceYRandom;
    }

    private function Drag(e:MouseEvent)
    {
        switch (e.type)
        {
            case 'mouseDown' :
                this.startDrag();
                this.addEventListener(MouseEvent.MOUSE_UP,Drag);
                break;

            case 'mouseUp' :
                this.stopDrag();
                this.removeEventListener(MouseEvent.MOUSE_UP,Drag);
                /*var m:*=this.parent;
                m.pos(this.x,this.y);*/

        }

    }
}
}

This is code in the main timeline.

//Global variables//
var imageDimension:Number = 360;
var gridType:Number = 6;
var puzzlePieceShape:String = "Sqaure";
var imageLoader:Loader = new Loader();
var bitmapArray:Array = [];
var puzzlePiece:PuzzlePiece;

var bitmapManip:BitmapManipulation;

loadImage();

function loadImage()
{
    imageLoader.load(new URLRequest("Mohanlal.jpg"));//The image being loaded is of 360*360
    imageHolder_mc.addChild(imageLoader);//imageHolder_mc is an empty MovieClip on stage
    imageHolder_mc.visible = false;
    imageLoader.contentLoaderInfo.addEventListener(Event.COMPLETE, layoutPieces);
}

function layoutPieces(evt:Event)
{
    bitmapManip = new BitmapManipulation(imageDimension,gridType);
    bitmapArray = bitmapManip.getBitmapImagePieces(imageHolder_mc);

    for (var j:uint =0; j<bitmapArray.length; j++)
    {

        for (var k:uint=0; k<bitmapArray[j].length; k++)
        {

            var bitmap:Bitmap = new Bitmap(bitmapArray[j][k]);
            puzzlePiece = new PuzzlePiece(400 * Math.random(),400 * Math.random());
            addChild(puzzlePiece);
            puzzlePiece.holder_mc.addChild(bitmap);

        }
    }
}

Bitmap Manipulation class

package 
{
    import flash.display.MovieClip;
    import flash.display.BitmapData;
    import flash.geom.Point;
    import flash.geom.Rectangle;

    public class BitmapManipulation extends MovieClip
        {
            private var imageDimension:Number;
            private var gridDimension:Number;

            public function BitmapManipulation(imageDimension:Number,gridDimension:Number)
            {
                this.imageDimension = imageDimension;
                this.gridDimension = gridDimension;

            }

            public function getBitmapImagePieces(imageMC:MovieClip):Array
            {
                var bitmapArray:Array = [];
                var imageBitmapData:BitmapData = new BitmapData(imageMC.width,imageMC.height);
                imageBitmapData.draw(imageMC);
                var tileDimesion:Number = this.imageDimension / this.gridDimension;

                for (var i:uint = 0; i<this.gridDimension; i++)
                {
                    bitmapArray[i] = new Array();

                    for (var j:uint = 0; j<this.gridDimension; j++)
                    {

                        var tempData:BitmapData = new BitmapData(tileDimesion,tileDimesion);
                        var tempRect:Rectangle = new Rectangle(((tileDimesion) * i),((tileDimesion) * j),tileDimesion,tileDimesion);
                        tempData.copyPixels(imageBitmapData,tempRect,new Point(0,0));
                        bitmapArray[i][j] = tempData;

                    }
                }

                return(bitmapArray);

            }
        }
    }

The puzzlepiece movieclip has two layers

Mask Layer - Two masks. One rectangular and one triangular in frame 1 and 2.
Holder Layer - holder_mc

I am trying to set the dimension of movieclips inside the puzzle piece using the code in PuzzlePiece class.

But I am getting this error.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at PuzzlePiece()[C:\Users\Shabeeb\Desktop\Puzzle OOP\PuzzlePiece.as:26]
    at PuzzlePiece_fla::MainTimeline/layoutPieces()[PuzzlePiece_fla.MainTimeline::frame1:33]

Line number 33 in main timeline class calls 

this.holder_mc.width = this.holder_mc.height = 60;
this.mask1_mc.width = this.mask2_mc.width = 60;
this.mask1_mc.height = this.mask2_mc.height = 60;

Is it wrong to access it like that. The PuzzlePiece is the export for a puzzle clip.

For the time being I am hard coding the dimension as 60. I have aloso uploaded the fla and as files.

https://rapidshare.com/files/4248268633/Puzzle_OOP.zip

This may shed some insight for you on what is going on http://www.developria.com/2010/04/combining-the-timeline-with-oo.html . (In a nutshell, you can't access objects that are declared on the timeline until the Flash player has actually created them.)

Note that I would not suggest that you use frame scripts that do anything more complicated than stop(), especially if you're also going to use document classes.

Thanks for that question Kodiak you have led me to the answer. I overcame the problem. I added the following to PuzzlePiece. I see that mask2_mc doesn't exist until frame 2.

this.gotoAndStop(1);
this.holder_mc.width = this.holder_mc.height = 60;
this.mask1_mc.width = 60;
this.mask1_mc.height = 60;
this.gotoAndStop(2);
this.mask2_mc.width = 60;
this.mask2_mc.height = 60

But I have a new problem now.

I need to select the mask on frame 1. I wrote a function like this in PuzzlePiece

public function selectFrame(frameNo:uint) {
this.gotoAndStop(frameNo);
}

I called the function from the fla with by adding a line this after addChild

puzzlePiece.selectFrame(1);

When I test I do not get an error but the I do not see the images loaded as bitmap and the puzzlePieces flicker probably due to jumping from 1 to 2 repeatedly.

How can I select the mask on frame 1.

Note:- In actual case I can have any number of masks in the timeline and I should be be able to select it from fla.