Cannot access a property

programming flash
1

Hi Everybody
I am working in flash cs3 past 1 years i have small error in flash cs3 coding.
I create external class file,

//
package {

import flash.display.MovieClip;
import flash.events.MouseEvent;
import flash.events.Event;
import flash.display.Stage;

public class Main extends MovieClip {
    private var iLoop:int=1;
    private var sta:Stage;
    public function Main():void {
        //sta=this.parent.parent;
        //trace(this.parent.parent);
        this.addEventListener(MouseEvent.CLICK,msDown);
        this.addEventListener(MouseEvent.ROLL_OUT,msOut);
        this.addEventListener(MouseEvent.ROLL_OVER,msOver);
    }
    public function msDown(e:MouseEvent):void {
        for (iLoop=1; iLoop<3; iLoop++) {
            //MovieClip(sta.getChildByName("mc"+iLoop)).gotoAndStop(1);
            MovieClip(getChildByName("mc"+iLoop)).gotoAndStop(1);
        }
        e.currentTarget.gotoAndStop(3);

    }
    public function msOut(e:MouseEvent):void {
        e.currentTarget.gotoAndStop(1);
    }
    public function msOver(e:MouseEvent):void {
        e.currentTarget.gotoAndStop(2);
    }
}

}

when i run this code i got the error message

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Main/msDown()

i attached my file also