In a new document when I create a movie clip on the stage and reference it in a script in the first keyframe, like aaa.scaleX = 0.2;, everything works fine.

But when I try this in the game I'm making, I get this error "1120: Access of undefined property aaa.". Why?

In my game I use a document class, here is the code that is executed at the beggining.

Actionscript Code:
package
{

    import flash.display.MovieClip;
    import flash.display.Stage;
    import flash.utils.*;
    import flash.events.TimerEvent;
    import flash.events.MouseEvent;
    import flash.ui.Mouse;
    import flash.display.Sprite;
    import flash.text.*;
    import flash.events.Event;
    import flash.events.KeyboardEvent;
    import flash.ui.Keyboard;
    import flash.media.SoundChannel;
    import flash.media.SoundTransform;

    public class Engine extends MovieClip
    {
        public var loopEventCounter:int; //for level events
        public var generalsCount:int = 0;

        public var enemies:Array = new Array  ;
        public var enemyGuns:Array = new Array  ;
        public var missiles:Array = new Array  ;
        public var shipmissiles:Array = new Array  ;
        public var bullets:Array = new Array  ;
        public var bulletexplosions:Array = new Array  ;
        public var bulletexplosions2:Array = new Array  ;
        public var bulletexplosionradius:Array = new Array  ;
        public var exhausts:Array = new Array  ;

        public var k:int;
        public var j:int;
        public var i:int;
        public var w:int;
        public var v:int;
        public var l:int; // menagement variable

        public var healthDisplay:HealthDisplay;
        public var redFlash:RedFlash;
        public var fadeOut:FadeOut;
        public var fadeIn:FadeIn;
        public var swarmIndicator:SwarmIndicator;
        public var ship:Ship;
        public var intro:Intro;

        public var playButtan:PlayButtan;
        public var mainMenu:MainMenu;
        public var options:Options;
        public var levelSelectionScreen:LevelSelectionScreen;
        public var deathMenu:DeathMenu;
        public var escMenu:EscMenu;
        public var debriefing:Debriefing;
        public var textFieldContainer:TextFieldContainer;

        public var gun:Gun;
        public var gun2:Gun;
        if(Vars.antiMissileTurret)
        {public var gun3:Gun;} //-------------------------------------------------------------------?????
        public var enemyGun:Gun;

        public var headLights:HeadLights;
        public var movingBackground:MovingBackground;
        public var movingBackgroundClouds:MovingBackgroundClouds;
        public var movingBackgroundClouds2:MovingBackgroundClouds2;
        public var exhaust:Exhaust;
        public var shipExhaust:ShipExhaust;

        public var radiance:Number = 180 / Math.PI;

        var enemy:Enemy;
        var missile:Missile;
        var shipMissile:Missile;
        var bullet:Bullet;
        var bulletExplosion:BulletExplosion;
        var bulletExplosion2:BulletExplosion2;
        public var bulletExplosionRadius:BulletExplosionRadius;

        var fpsCounter:FPSCounter;

        public var gameTimer:Timer;
        public var gameTimerSlow:Timer;
        public var dying:Timer = new Timer(6000); //DIE TIME
        /*public var delayTextFade:Timer;
        public var textFadeRepeater:Timer;*/
        public var delayTextFade;
        public var textFadeRepeater;
        public var bulletTimeInterval;

        public var mouseButtonIsBeingPressed:Boolean;
        public var spaceBarIsBeingPressed:Boolean;
        public var missileReloadSoundPlayed:Boolean;

        public var ticksSinceLastShot:int;
        public var ticksSinceBulletTime:int = 100;
        public var ticksSinceLastAntiShot:int;
        public var enemyMissilesAmount:int;
        public var enemyGunsAmount:int;
        public var activeBarrel:int = 1;

        //
        public var eventTimer:int = 1;
        public var microEventTimer:int = 1;
        //

        public var bulletHit:BulletHit;
        public var bulletHit2:BulletHit2;
        public var bulletHit3:BulletHit3;
        public var bulletHit4:BulletHit4;
        public var bulletHit5:BulletHit5;
        public var bulletHit6:BulletHit6;
        public var machineGun:MachineGun;
        public var machineGun2:MachineGun2;
        public var machineGun3:MachineGun3;
        public var missilesReload:MissilesReload;
        public var missileLaunch:MissileLaunch;
        public var lowExplosion:LowExplosion;
        public var smallExplosion:SmallExplosion;

        public var sfxSoundChannel:SoundChannel;
        public var soundTransformMute = new SoundTransform  ;
        public var soundTransformVolumeControl = new SoundTransform  ;

        public function Engine () // ------------------------------------------------- ENGINE START
        {
            loadEngine(); //some basic functions

        }

                 public function loadEngine ()
        {
            addEventListener(Event.ADDED_TO_STAGE,onAddToStage,false,0,true);

            sfxSoundChannel = new SoundChannel  ;
            soundTransformMute.volume = 0;
            soundTransformVolumeControl.volume = 0.3;

            mouseButtonIsBeingPressed = false;
            spaceBarIsBeingPressed = false;
        }