TypeError, cant figure it out

programming flash
1

TypeError: Error #1009: Cannot access a property or method of a null object reference.

I know everyone hates these errors, but i cant seem to get rid of it.
I’m working on a project for school that includes pathfinding, the pathfinding part is all done but now for the tweening to make the character (Doctor) walk the path. My solution to this came up like this (its probably not perfect yet but hey, I’m still in school)

the function that starts filling the array with movement tweens (I’m not quite sure if this will work, maybe thats what’s causing the error)

        function Move(event:MouseEvent):void {
                    path=$grid.findPath("n1","n6");
                    
                    for (var i:int = 0; i <path._path.length; i++) {
                    var nextNode=$grid.getNodeById(path._path*);
                    trace(nextNode);
                    var tx=nextNode.x;
                    var ty=nextNode.y;
                    trace(tx);
                    var dx:Number=tx-Doctor.x;
                    var dy:Number=ty-Doctor.y;
                    var dis:Number=Math.sqrt(dx*dx+dy*dy);
                    //how long will it take
                    var time:Number=dis/speed;
                    //if (dis!=0) {
                        tweenArrayX*=new Tween(Doctor,"x",None.easeNone,Doctor.x,tx,time,false);
                        tweenArrayY*=new Tween(Doctor,"y",None.easeNone,Doctor.y,ty,time,false);
                    //}
                    if (i==path._path.length) {
                        startMoving=true;
                    }
                }
        }

the Update function

        function Update(event:Event):void {
                
                if (startMoving== true){
                if (!moving) {
                    
                    for (var u:int = 0; u <tweenArrayX.length; u++) {
                        tweenerX=tweenArrayX[u];
                        tweenerY=tweenArrayY[u];
                        tweenerX.addEventListener(TweenEvent.MOTION_FINISH, MoveComplete);
                        moving=true;
                        
                        if (u == tweenArrayX.length)
                        {
                            startMoving=false;
                        }
                    }
                }
                }
            
        }

The error is in the

tweenArrayX*=new Tween(Doctor,"x",None.easeNone,Doctor.x,tx,time,false);
                        tweenArrayY*=new Tween(Doctor,"y",None.easeNone,Doctor.y,ty,time,false);

part

I’d be really glad if someone can help me out on this! or even help me improve it!

**
sorry it’s a bit messy, but i haven’t gotten round to cleaning up yet.
**