Problem removing child at end game screen

Question

Hi everyone
I am creating an android app for my university corusework and it is based on shooting alien spaceships as they fly in from the right side of the screen. When my gun is shot enough times the game should go to the end screen, the problem is I need to remove the alien bullets as the game ends. However I keep getting an error of an undefined property. Here is the code for my main timeline:
import flash.events.MouseEvent;
import flash.events.Event;
import flash.display.MovieClip;
//gun control
var upButtonIsDown:Boolean;
var downButtonIsDown:Boolean;
var livesLost:Number = 0;
stop();
btn_up.addEventListener(MouseEvent.MOUSE_DOWN,upDown);
btn_down.addEventListener(MouseEvent.MOUSE_DOWN,downDown);
btn_up.addEventListener(MouseEvent.MOUSE_UP,upUp);
btn_down.addEventListener(MouseEvent.MOUSE_UP,downUp);
//bg
var bgButtonIsDown:Boolean;
bg.addEventListener(MouseEvent.MOUSE_UP,upUp);
bg.addEventListener (MouseEvent.MOUSE_UP,downUp);
stage.addEventListener(Event.ENTER_FRAME,pulse);
function pulse(evt:Event)
          if (currentFrame!=2) return;
          if (upButtonIsDown) mc_gun.y-=5;
          if (downButtonIsDown) mc_gun.y+=5;
          checkforHits();
function checkforHits()
          for (var i:Number=0;i<numChildren;i++)
                               if (getChildAt  is alienbullet)
                                         var ab:alienbullet = alienbullet (getChildAt (i));
                                         if (mc_gun.hitTestObject(ab) && ab.currentFrame==1)
                                                   mc_gun.gotoAndStop(mc_gun.currentFrame+1);
                                                   ab.gotoAndStop(2);
                                                   livesLost++;
                                                   if (livesLost>3) endGame();
                                                   return;
function endGame()
          for (var i:Number=numChildren-1;i>=0;i--)
                    if (getChildAt(i) is alienbullet)
                              var ab:alienbullet = alienbullet(getChildAt(i));
                              ab.killMe();
          gotoAndStop(21);
function upDown (evt:MouseEvent)
          upButtonIsDown=true;
function downDown (evt:MouseEvent)
          downButtonIsDown=true;
function upUp (evt:MouseEvent)
          upButtonIsDown=false;
function downUp (evt:MouseEvent)
          downButtonIsDown=false;
//shoot code
mc_gun.addEventListener(MouseEvent.MOUSE_DOWN,shoot);
stage,addEventListener(Event.ENTER_FRAME,alienhit);
var bulletArray:Array = new Array();
bulletArray.push(mc_bullet1);
bulletArray.push(mc_bullet2);
bulletArray.push(mc_bullet3);
bulletArray.push(mc_bullet4);
bulletArray.push(mc_bullet5);
bulletArray.push(mc_bullet6);
var bulletCounter:Number = 0;
function alienhit(evt:Event)
          for (var i:Number=0;i<bulletArray.length;i++)
                              if (mc_alienA1.hitTestObject(bulletArray[i])) trace("alienA1 has been hit" +i);
                              if (mc_alienA2.hitTestObject(bulletArray[i])) trace("alienA2 has been hit" +i);
                              if (mc_alienA3.hitTestObject(bulletArray[i])) trace("alienA3 has been hit" +i);
function shoot(evt:MouseEvent)
          if (bulletCounter<5) bulletCounter++;
          else bulletCounter=0;
          shootmc(bulletArray[bulletCounter]);
function shootmc(mc:MovieClip)
          mc.visible=true;
          mc.x = mc_gun.x;
          mc.y = mc_gun.y;
          mc.gotoAndPlay(2);
Now the endGame function is supposed to call on this code which is in the package file (script for aliens)
public function killMe()
                                        this.removeEventListener(Event.ENTER_FRAME,pulse);
                                        parent.removeChild(this);
full script:
package
          import flash.display.MovieClip;
          import flash.events.Event;
          public class alien1 extends MovieClip
                    var yMin = 0;
                    var yMax = stage.stageHeight;
                    var yInc = 10;
                    var maxPulses= Math.round(yMax/yInc);
                    var changeDirectionAt = Math.round(Math.random() * maxPulses);
                    var pulseCount = 0;
                    var fireCount=0;
                    var fireAt = Math.round(Math.random() * maxPulses);
                    public function alien1()
                              this.addEventListener(Event.ENTER_FRAME,apulse);
                    function apulse(evt:Event)
                              pulseCount++;
                              fireCount++;
                              if (pulseCount>=changeDirectionAt)
                                        yInc *=  -1;
                                        pulseCount = 0;
                                        changeDirectionAt = Math.round(Math.random() * maxPulses);
                                        return;
                              if (fireCount>=fireAt)
                                        var bullet:alienbullet = new alienbullet();
                                        bullet.x = this.x;
                                        bullet.y = this.y;
                                        parent.addChild(bullet);
                                        fireCount=0;
                                        fireAt = Math.round(Math.random() * maxPulses);
                              this.y +=  yInc;
                              if (this.y <= yMin)
                                        yInc *=  -1;
                              if (this.y >= yMax)
                                        yInc *=  -1;
                              public function killMe()
                                        this.removeEventListener(Event.ENTER_FRAME,pulse);
                                        parent.removeChild(this);
Would really appreciate any help! Thanks

Answer 1

if (mc_alienA1.hitTestObject(bulletArray[i])) trace("alienA1 has been hit" +i);
this is line 103 and is in the main timeline
another error that shows up is
TypeError: Error #1009: Cannot access a property or method of a null object reference.
          at alien1/apulse()[C:\Users\_____\Desktop\DES B\alien1.as:46]
                                        var bullet:alienbullet = new alienbullet();
                                        bullet.x = this.x;
                                        bullet.y = this.y;
                                        parent.addChild(bullet); <<< line 46
                                        fireCount=0;
                                        fireAt = Math.round(Math.random() * maxPulses);