I need help with my school project

Hi, I need help with a school project, as I have a little amount of knowledge and I'm quite inexperienced with programming. I will put the error beneath the code, thanks for your time.

stop();

var dx:Number;
var dy:Number;
var cCounter:int=0;
var xSpeed:Number=10;
var stageWidth:Number=stage.stageWidth;
var stageHeight:Number=stage.stageHeight;
var x_coordinate:Number=x;
var y_coordinate:Number=y;
var speed:Number=15;

function randRange(minNum:Number, maxNum:Number):Number {
return (Math.floor(Math.random()*(maxNum - minNum + 1)) + minNum);
}

stage.addEventListener(Event.ENTER_FRAME, checkMouse);
function checkMouse(evt:Event):void {
dx=mouseX-gun.x;
dy=mouseY-gun.y;
gun.rotation = (Math.atan2(dy, dx) * 180 / Math.PI);
}

stage.addEventListener(MouseEvent.CLICK, shotFired);
function shotFired(Mouse:Event):void {
var bulletMC:MovieClip = new bullet_mc();
cCounter++;
bulletMC.x=gun.x;
bulletMC.y=gun.y;
bulletMC.rotation=gun.rotation;
bulletMC.speed=15;
bulletMC.name="bullet_mc"+cCounter;
bulletMC.addEventListener(Event.ENTER_FRAME, BulletMoveHandler);
addChild(bulletMC);
}

function BulletMoveHandler(event:Event):void {
var bulletMC:MovieClip=event.currentTarget as MovieClip;
bulletMC.x=bulletMC.x+Math.cos(bulletMC.rotation/180*Math.PI)*speed;
bulletMC.y=bulletMC.y+Math.sin(bulletMC.rotation/180*Math.PI)*speed;
if (bulletMC.x>stageWidth||bulletMC.y>stageHeight) {
  removeChild(bulletMC);
  bulletMC.removeEventListener(Event.ENTER_FRAME, BulletMoveHandler);
}
}

function createEnemy():void {
if (randRange(0,25)==0) {
  var enemyMC:MovieClip = new enemy_mc();
  cCounter++;
  x_coordinate= - enemyMC.width;
  y_coordinate=randRange(stageHeight-enemyMC.height,215);
  enemyMC.x=x_coordinate;
  enemyMC.y=y_coordinate;
  enemyMC.speed=xSpeed;
  enemyMC.name="Enemy"+cCounter;
  enemyMC.addEventListener(Event.ENTER_FRAME, EnemyMoveHandler);
  addChild(enemyMC);
}
if (enemyMC.hitTestObject(bulletMC)) {
  var bulletMC:MovieClip=bullet_mc(0);
  removeChild(enemyMC);
  enemyMC.removeEventListener(Event.ENTER_FRAME, EnemyMoveHandler);
}
}

function EnemyMoveHandler(event:Event):void {
var enemyMC:MovieClip=event.currentTarget as MovieClip;
enemyMC.x+=enemyMC.speed;

if (enemyMC.x>stageWidth) {
  removeChild(enemyMC);
  enemyMC.removeEventListener(Event.ENTER_FRAME, EnemyMoveHandler);
}
}

cursor_mc.startDrag(true);
Mouse.hide();

stage.addEventListener(Event.ENTER_FRAME, main);
function main(event:Event):void {
createEnemy();
}
for each(var enemy:enemy_mc in enemy_mc){
for each(var bullet:bullet_mc in bullet_mc){
  if(bullet.x > enemy.x - enemy.width / 2 &&
     bullet.x < enemy.x + enemy.width / 2 &&
     bullet.y > enemy.y - enemy.height / 2 &&
     bullet.y < enemy.y + enemy.width / 2){
   trace("collision");
  }
}
}

Error -

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

at Flash001_Shooting_fla::MainTimeline/createEnemy()

at Flash001_Shooting_fla::MainTimeline/main()

TypeError: Error #2007: Parameter hitTestObject must be non-null.

at flash.display::DisplayObject/_hitTest()

at flash.display::DisplayObject/hitTestObject()

at Flash001_Shooting_fla::MainTimeline/createEnemy()

at Flash001_Shooting_fla::MainTimeline/main()