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

var score:Number = 0;
var objectoriginalX2:Number;
var objectoriginalY2:Number;

this.stop();

author.buttonMode = true;
author.addEventListener(MouseEvent.MOUSE_DOWN, pickupObject2);
author.addEventListener(MouseEvent.MOUSE_UP, dropObject2);

doi.buttonMode = true;
doi.addEventListener(MouseEvent.MOUSE_DOWN, pickupObject2);
doi.addEventListener(MouseEvent.MOUSE_UP, dropObject2);

year.buttonMode = true;
year.addEventListener(MouseEvent.MOUSE_DOWN, pickupObject2);
year.addEventListener(MouseEvent.MOUSE_UP, dropObject2);

page.buttonMode = true;
page.addEventListener(MouseEvent.MOUSE_DOWN, pickupObject2);
page.addEventListener(MouseEvent.MOUSE_UP, dropObject2);

volumn.buttonMode = true;
volumn.addEventListener(MouseEvent.MOUSE_DOWN, pickupObject2);
volumn.addEventListener(MouseEvent.MOUSE_UP, dropObject2);

journal.buttonMode = true;
journal.addEventListener(MouseEvent.MOUSE_DOWN, pickupObject2);
journal.addEventListener(MouseEvent.MOUSE_UP, dropObject2);

article.buttonMode = true;
article.addEventListener(MouseEvent.MOUSE_DOWN, pickupObject2);
article.addEventListener(MouseEvent.MOUSE_UP, dropObject2);

function pickupObject2(event:MouseEvent):void {
event.target.startDrag(true);
event.target.parent.addChild(event.target);
objectoriginalX2 = event.target.x;
objectoriginalY2 = event.target.y;
}

function dropObject2(event:MouseEvent):void {
event.target.stopDrag();
var matchingTargetName:String = "target" + event.target.name;
var matchingTarget:DisplayObject = getChildByName(matchingTargetName);
if (event.target.dropTarget != null && event.target.dropTarget.parent == matchingTarget){
  event.target.removeEventListener(MouseEvent.MOUSE_DOWN, pickupObject);
  event.target.removeEventListener(MouseEvent.MOUSE_UP, dropObject);
  event.target.buttonMode = false;
  event.target.x = matchingTarget.x;
  event.target.y = matchingTarget.y;
 
  response_mc.gotoAndStop(1);
  nextquestion.gotoAndStop(1);
 
  score++
  if ( score==7){
   response_mc.gotoAndStop(2);
   nextquestion.gotoAndStop(2);
   nextquestion.addEventListener(MouseEvent.CLICK, question2);
   function question2(event:MouseEvent):void{
    gotoAndStop(3);
    page.gotoAndStop(2);
    volumn.gotoAndStop(2);
    article.gotoAndStop(2);
    journal.gotoAndStop(2);
    year.gotoAndStop(2);
    author.gotoAndStop(2);
    doi.gotoAndStop(2);
   }
   }
 

} else {
  event.target.x = objectoriginalX2;
  event.target.y = objectoriginalY2;
}

   
}

I use the debug to track.

it warns me that this code has error  page.gotoAndStop(2);
but I don't know what is the error.

It has frame2. all the objects have frame2. But debug warns all the objects such as page,volumn,article,journal,year,author,doi