Another stage question

programming flash
1

I’m still not understanding how to access the stage within a class.

basically all I want to do is get the midpoint with stageWidth and stageHeight but I keep getting this error:

Cannot access a property or method of a null object reference.

code as it stands:

package com.mysite.mouse{
    //imports
    import flash.display.MovieClip;
    import flash.display.Stage;
    import flash.events.MouseEvent;
    import flash.geom.Point;


    //class
    public class ZoomToPoint extends MovieClip {
        //variables
        private var clip:MovieClip;
        private var scp:Point;
        //constructor
        public function ZoomToPoint($clip:MovieClip) {

            clip = $clip;
            clip.buttonMode = true;

            clip.addEventListener(MouseEvent.CLICK, clickHandle);

        }
        //handler
        private function clickHandle(evt:MouseEvent):void {

            scp = new Point(stage.stageWidth/2,stage.stageHeight/2);
        }
    }
}