package {
    import flash.display.MovieClip;
    import flash.display.Sprite;
    import flash.events.Event;
    import flash.events.KeyboardEvent;
    import flash.events.TimerEvent;
    import flash.utils.Timer;
    import flash.ui.Keyboard;
    public class Main extends Sprite {
        var player:Player;
        var enemy:Enemy;
        var checkTimer:Timer = new Timer(20);

        public function Main() {
            this.addEventListener(Event.ADDED_TO_STAGE,onAddedToStage);
        }
        private function onAddedToStage(event:Event):void {
            this.removeEventListener(Event.ADDED_TO_STAGE,onAddedToStage);
            this.addEventListener(Event.ENTER_FRAME,setupGame);
        }
        public function setupGame(e:Event) {
            spawnPlayer();
            spawnEnemy();
            checkTimer.start();
            this.removeEventListener(Event.ENTER_FRAME,setupGame);
            checkTimer.addEventListener(TimerEvent.TIMER, collisionCheck);
        }
        public function spawnPlayer() {

            player=new Player  ;
            addChild(player);
            player.x=225;
            player.y=200;
            player.gotoAndStop(1);
            player.weapon.gotoAndStop(1);
        }
        public function spawnEnemy() {
            enemy=new Enemy ;
            addChild(enemy);
            enemy.x=25;
            enemy.y=50;
        }
        public function collisionCheck(Event:TimerEvent) {
            if (enemy.hitTestObject(player)) {
                removeChild(enemy);
                trace("hit");
            }
        }
    }
}

ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.
    at flash.display::DisplayObjectContainer/removeChild()
    at Main/collisionCheck()
    at flash.utils::Timer/_timerDispatch()
    at flash.utils::Timer/tick()