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

[5TonsOfFlax]

You could, but I can't open it here. If it's CS4, I can't open it at all.

I'd suggest keeping references to tn, ap, and cw at the class level, and making a function to set up the targets and mouse-up listeners. Call that function from the frame where the targets actually exist.

package
{
    import flash.display.MovieClip;
    import flash.events.MouseEvent;
    import flash.display.DisplayObject;
    import DragDrop;
    import TN;
    import AP;
    import CW;


    
    public class DragGame extends MovieClip
    {
        private var tn:TN;
        private var ap:AP;
        private var cw:CW;


        
        public function DragGame()
        {
            createPieces();
            
        }
        
        private function createPieces():void
        {
            tn = new TN();
            addChild(tn);
            tn.x = 169.9;
            tn.y = 90.8;
            tn.width = 370.9;
            tn.height = 68;
            tn.alpha = 0.3;
            
            ap = new AP();
            addChild(ap);
            ap.x = 496.6;
            ap.y = 169.9;
            ap.width = 162.8;
            ap.height = 110.2;
            ap.alpha = 0.3;
            
            cw = new CW();
            addChild(cw);
            cw.x = 44.8;
            cw.y = 193.1;
            cw.width = 197.0;
            cw.height = 94;
            cw.alpha = 0.3;
        }
        
       public function setUpTargets():void{
            tn._targetPiece = ttn_mc;
            tn.addEventListener(MouseEvent.MOUSE_UP, checkTarget);
            trace(ttn_mc);

            ap._targetPiece = tap_mc;
            ap.addEventListener(MouseEvent.MOUSE_UP, checkTarget);
            trace(tap_mc);

            cw._targetPiece = tcw_mc;
            cw.addEventListener(MouseEvent.MOUSE_UP, checkTarget);
            trace(tcw_mc);
       }

       private function checkTarget(event:MouseEvent):void
       {
           var ec:MovieClip = MovieClip(event.currentTarget);
           var target:DisplayObject = DisplayObject(ec._targetPiece);
           if(ec.hitTestObject(target))
           {
               trace("whooooot!");
               ec.x = target.x;
               ec.y = target.y;
               ec.removeEventListener(MouseEvent.MOUSE_UP, checkTarget);
               ec.disable();
           }
           else
           {
               ec.x = ec._origX;
               ec.y = ec._origY;
           }
       }
   }
    
} 

call setUpTargets from the frame where the targets exist.