Small OOP Problem

programming flash
1

Hi there,

I am still very new to OOP and writing custom classes. Learning alot as I am going along. I am trying to write a simple preloader class. My question in my constructor function I call the drawshape() method as code below shows:

package
{
	import flash.display.MovieClip;
	import flash.display.Graphics;
	import flash.display.Shape;
	import flash.display.Sprite;
	
	public class Preloader extends MovieClip
	{
		
		var _shape:Shape;
		
		public function Preloader()
		{
			drawShape();
		}
		
		private function drawShape():void
		{
			_shape = new Shape();
			_shape.graphics.lineStyle(2,0xff0000);
			_shape.graphics.beginFill(0x00ff00);
			_shape.graphics.drawRect(stage.stageWidth/2 , stage.stageHeight/2, 20, 30);
			_shape.graphics.endFill();
			addChild(_shape);
		}
	}
}

on my timeline frame 1 I create an instance of the class:

var loader:Preloader = new Preloader();

When running the movie I get the following error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at Preloader/::drawShape()
	at Preloader$iinit()
	at loader_fla::MainTimeline/loader_fla::frame1()

I understand that the onject has not been created when I call it, but what do I do to prevent this? I am still very new at AS3, and it has scared me since release. :);(

Thanks