Communicating between classes

1

Hi I’m new to AS3, I hope you guys can help me. I have this error when I run the files.
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at com.common::stageConfig()
at cloudsMain()

CloudsMain.as which is attached to the .fla


package {
    import flash.display.MovieClip;
    import com.common.stageConfig;
    public class cloudsMain extends MovieClip {
        public var _stageConfig:stageConfig;
        public function cloudsMain () {
            _stageConfig = new stageConfig ();
        }

    }
}

The external class which is called.


package com.common{
    import flash.display.MovieClip;
    import flash.display.StageAlign;
    import flash.display.StageScaleMode;
    import flash.display.DisplayObject;
    import flash.events.Event;
    public class stageConfig extends MovieClip {
        public function stageConfig ():void {
            stage.scaleMode = StageScaleMode.NO_SCALE;
            stage.align = StageAlign.TOP_LEFT;
        }
        public function centralisedDO (_do:DisplayObject):void {
            _do.x = stage.stageWidth / 2;
            _do.y = stage.stageHeight / 2;
            trace ("centralised "+_do+" at "+_do.x+","+_do.y);
        }
    }
}

Basically I’m trying to create a class that I can call and centralise any mc on stage by eg.
stageConfig.centralisedDO(my_mc), However I encounter the above error.

Please advice.