How to make a document class visible through button click.

package

{

          import flash.display.MovieClip;

          import flash.display.Sprite;

          import flash.events.MouseEvent;

          import flash.events.Event;

          import flash.display.Graphics;

          /**

           * ...

           * @7111211 Alex

           */

          public class Toggle extends MovieClip

          {

                    private var button:MovieClip;

                    private var button2:MovieClip;

                    public var myClock:AnalogueClock;

                    public function Toggle():void

          {

                    button = new MovieClip();

                    button.graphics.beginFill(0xFFCC00);

                    button.graphics.drawRect(0, 0, 80, 20);

                    button.graphics.endFill();

                    button.useHandCursor = true;

                    button.buttonMode = true;

                    button.mouseChildren = false;

                    button2 = new MovieClip();

                    button2.graphics.beginFill(0xCC0000);

                    button2.graphics.drawRect(0, 0, 80, 20);

                    button2.graphics.endFill();

                    button2.useHandCursor = true;

                    button2.buttonMode = true;

                    button2.mouseChildren = false;

                    button.addEventListener(MouseEvent.CLICK, buttonClickHandler);

                    button2.addEventListener(MouseEvent.CLICK, buttonClickHandler2);

                    this.addChild(button)

                    this.addChild(button2)

                    this.addChild(myClock)

          }

          public function buttonClickHandler(event:MouseEvent):void

          {

                    this.button.visible = false;

                    this.button2.visible = true;

                    this.myClock.visible = false;

          }

          private function buttonClickHandler2(event:MouseEvent):void

          {

                    this.button.visible = true;

                    this.button2.visible = false;

                    this.myClock.visible = true;

          }

          }

}

Comes up with an error saying the parameter child must be non-null, dont really know whats going on any help will be greatly appreciated