Error #2007: Parameter hitTestObject must be non-null

Hi, im still really new to actionscript 3.0 and try to make a simple drag and drop picture puzzle game but this error keeps on coming.. i tested it in another file and the game works just fine but when I input it in a new scene in the original file.. where the game main menu, opening, etc is at, the error keeps on coming when I  drop the puzzle piece to the designated target.. what should I do?

stop();

var objectoriginalX:Number;
var objectoriginalY;Number;
import flash.display.DisplayObject;

Multitouch.inputMode = MultitouchInputMode.TOUCH_POINT;

puzzle1piece1.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece1.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

puzzle1piece2.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece2.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

puzzle1piece3.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece3.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

puzzle1piece4.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece4.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

puzzle1piece5.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece5.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

puzzle1piece6.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece6.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

puzzle1piece7.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece7.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

puzzle1piece8.addEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
puzzle1piece8.addEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);

var fl_DragBounds:Rectangle = new Rectangle(0, 0, stage.stageWidth, stage.stageHeight);

function fl_TouchBeginHandler(event:TouchEvent):void
{
event.target.startTouchDrag(event.touchPointID, false, fl_DragBounds);
objectoriginalX = event.target.x;
objectoriginalY = event.target.y;

}

function fl_TouchEndHandler(event:TouchEvent):void
{
event.target.stopTouchDrag(event.touchPointID);
var matchingTargetName:String = event.target.name + "target";
var matchingTarget:DisplayObject = getChildByName(matchingTargetName);
if(event.target.dropTarget != null && event.target.hitTestObject(matchingTarget))
{
event.target.removeEventListener(TouchEvent.TOUCH_BEGIN, fl_TouchBeginHandler);
event.target.removeEventListener(TouchEvent.TOUCH_END, fl_TouchEndHandler);
event.target.x = matchingTarget.x;
event.target.y = matchingTarget.y;
}
if(
puzzle1piece1.x == puzzle1piece1target.x && puzzle1piece1.y == puzzle1piece1target.y &&
puzzle1piece2.x == puzzle1piece2target.x && puzzle1piece2.y == puzzle1piece2target.y &&
puzzle1piece3.x == puzzle1piece3target.x && puzzle1piece3.y == puzzle1piece3target.y &&
puzzle1piece4.x == puzzle1piece4target.x && puzzle1piece4.y == puzzle1piece4target.y &&
puzzle1piece5.x == puzzle1piece5target.x && puzzle1piece5.y == puzzle1piece5target.y &&
puzzle1piece6.x == puzzle1piece6target.x && puzzle1piece6.y == puzzle1piece6target.y &&
puzzle1piece7.x == puzzle1piece7target.x && puzzle1piece7.y == puzzle1piece7target.y &&
puzzle1piece8.x == puzzle1piece8target.x && puzzle1piece8.y == puzzle1piece8target.y
)
{
gotoAndPlay(2);
}
}