Could somebody explain why I am getting this Error #1009 message?

Question

Hi. I am following a tutorial on lynda.com in hopes of learning how to use actionscript 3.0 and my copy of flash CS4 to create games. I wrote my code exactly like it showed in the tutorial, but for some reason I will occasionally receive an error that says
"TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Monster/die()
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at fl.motion::AnimatorBase/end()
at fl.motion::AnimatorBase/handleLastFrame()
at fl.motion::AnimatorBase/nextFrame()
at fl.motion::AnimatorBase/handleEnterFrame()"
It doesn't happen all the time but it will every once in a while. There doesn't really seem to be anything specifically that triggers it either. I also happened to notice that not all of the enemies respond to being clicked on. I don't know if these two things are related or not. I don't know if the error is caused by the code for the monster library item or the code for the main timeline. Thank you in advance for looking at my code.
This is my code for the monster library item:
import fl.motion.Animator;
import fl.motion.MotionEvent;
var this_xml:XML = <Motion duration="30" xmlns="fl.motion.*" xmlns:geom="flash.geom.*" xmlns:filters="flash.filters.*">
<source>
  <Source frameRate="10" x="275" y="200" scaleX="1" scaleY="1" rotation="0" elementType="movie clip" symbolName="monster">
   <dimensions>
    <geom:Rectangle left="-32.5" top="-32.95" width="65.05" height="65.95"/>
   </dimensions>
   <transformationPoint>
    <geom:Point x="0.49961568024596464" y="0.49962092494313876"/>
   </transformationPoint>
  </Source>
</source>
<Keyframe index="0" tweenSnap="true" tweenSync="true">
  <tweens>
   <SimpleEase ease="0"/>
  </tweens>
</Keyframe>
<Keyframe index="29" tweenSnap="true" tweenSync="true" scaleX="2.782" scaleY="2.697">
  <tweens>
   <SimpleEase ease="0"/>
  </tweens>
</Keyframe>
</Motion>;
var this_animator:Animator = new Animator(this_xml, this);
this_animator.play();
this_animator.addEventListener(MotionEvent.MOTION_END, hurtPlayer);
function hurtPlayer(MotionEvent):void
var main:MovieClip = MovieClip(this.parent.parent);
main.decreaseEnergy();
this.parent.removeChild(this);
this.addEventListener(MouseEvent.CLICK, killMonster);
function killMonster(event:MouseEvent):void
this_xml = <Motion duration="5" xmlns="fl.motion.*" xmlns:geom="flash.geom.*" xmlns:filters="flash.filters.*">
<source>
  <Source frameRate="10" x="275" y="200" scaleX="1" scaleY="1" rotation="0" elementType="movie clip" symbolName="monster" class="Monster">
   <dimensions>
    <geom:Rectangle left="-32.5" top="-32.95" width="65.05" height="65.95"/>
   </dimensions>
   <transformationPoint>
    <geom:Point x="0.49961568024596464" y="0.49962092494313876"/>
   </transformationPoint>
  </Source>
</source>
<Keyframe index="0" rotateDirection="cw" rotateTimes="1" tweenSnap="true" tweenSync="true">
  <tweens>
   <SimpleEase ease="0"/>
  </tweens>
</Keyframe>
<Keyframe index="4" rotateDirection="cw" rotateTimes="1" tweenSnap="true" tweenSync="true" scaleX="0.416" scaleY="0.378">
  <color>
   <Color alphaMultiplier="0"/>
  </color>
  <tweens>
   <SimpleEase ease="0"/>
  </tweens>
</Keyframe>
</Motion>;
this_animator = new Animator(this_xml, this);
this_animator.play();
this_animator.addEventListener(MotionEvent.MOTION_END, die);
function die(event:MotionEvent):void
var main:MovieClip = MovieClip(this.parent.parent);
main.increaseScore();
this_animator.removeEventListener(MotionEvent.MOTION_END, hurtPlayer);
this.parent.removeChild(this);
This is my code for the main timeline:
var monstersInGame:uint;
var monsterMaker:Timer;
var container_mc:MovieClip;
var cursor:MovieClip;
var score:int;
var energy:int;
function initializeGame():void
monstersInGame = 10;
monsterMaker = new Timer(1000, monstersInGame);
container_mc = new MovieClip;
addChild(container_mc);
monsterMaker.addEventListener(TimerEvent.TIMER, createMonsters);
monsterMaker.start();
cursor = new Cursor();
addChild(cursor);
cursor.enabled = false;
Mouse.hide();
stage.addEventListener(MouseEvent.MOUSE_MOVE, dragCursor);
score = 0;
energy = energy_mc.totalFrames;
energy_mc.gotoAndStop(energy);
function dragCursor(event:MouseEvent):void
cursor.x = this.mouseX;
cursor.y = this.mouseY;
function createMonsters(event:TimerEvent):void
var monster:MovieClip;
monster = new Monster();
monster.x = Math.random() * stage.stageWidth;
monster.y = Math.random() * stage.stageHeight;
container_mc.addChild(monster);
function increaseScore():void
score ++;
if(score >= monstersInGame)
  monsterMaker.stop();
function decreaseEnergy():void
energy --;
if(energy <= 0)
  monsterMaker.stop();
else
  energy_mc.gotoAndStop(energy);
initializeGame();

Answer 1

Ok. Apparently, I didn't test it enought times to get the error. I went back and tested it again to get the error. When I did this, the
trace(this); revealed [object Monster]
and
trace(this.parent); revealed [object MovieClip].
The only one that was different was the one that gave the error. For that one the
trace(this); revealed [object Monster]
and
trace(this.parent); revealed null.
Then it gave all of the error information.