TypeError 1009 & addEventListener

[couto607]

I’m fairly new to Actionscript 3.0, and I’m having some trouble with this error. I’m currently setting up a small Space Shooter game and am stuck at this error. All was working fine until I started to code the movement class.

Error:

GameScreen
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at GameScreen()
    at Main/showMenu()
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at TitleScreen/clickPlay()

[CENTER]Code[/CENTER]
Main(this is the document class):

package
{
    import flash.display.MovieClip;
    import flash.events.Event;
    
    public class Main extends MovieClip
    {
        public var screen_mc:MovieClip;
        
    
        public function Main()
        {
            screen_mc = new TitleScreen();
            screen_mc.addEventListener(TitleScreen.PLAY, showMenu); //folder ScreenTitle.<here>
            screen_mc.addEventListener(TitleScreen.GAMEOPTIONS, showOps);
            addChild(screen_mc);
        }
        
        public function showMenu(e:Event) //from dispatch (new Event)
        {
            screen_mc.removeEventListener(TitleScreen.PLAY, showMenu);
            removeChild(screen_mc);
            screen_mc = new GameScreen();
            screen_mc.addEventListener(GameScreen.PAUSE, gamePause);
            screen_mc.addEventListener(GameScreen.WIN, gameWin);
            screen_mc.addEventListener(GameScreen.LOSE, gameLose);
            addChild(screen_mc);    
        }
        
        public function showOps(e:Event)
        {
            screen_mc.removeEventListener(TitleScreen.GAMEOPTIONS, showMenu);
            removeChild(screen_mc);
            screen_mc = new OptionsScreen();
            addChild(screen_mc);    
        }
        
        public function gamePause(e:Event)
        {
            screen_mc.removeEventListener(GameScreen.PAUSE, showMenu);
            removeChild(screen_mc);
            screen_mc = new PauseMenu();
            addChild(screen_mc);    
        }
        public function gameWin(e:Event)
        {
            screen_mc.removeEventListener(GameScreen.WIN, gameWin);
            removeChild(screen_mc);
            screen_mc = new WinScreen();
            addChild(screen_mc);    
        }
        public function gameLose(e:Event)
        {
            screen_mc.removeEventListener(GameScreen.LOSE, gameLose);
            removeChild(screen_mc);
            screen_mc = new LoseScreen();
            addChild(screen_mc);    
        }
    }
}

TitleScreen.as

package  
{
    import flash.display.MovieClip;
    import flash.events.Event;
    import flash.events.MouseEvent;
    
    public class TitleScreen extends MovieClip
    {
        public static const PLAY:String = "play";
        public static const GAMEOPTIONS:String = "option";
        
        public function TitleScreen() 
        {
            startMC.addEventListener( MouseEvent.CLICK, clickPlay);
            optionsMC.addEventListener( MouseEvent.CLICK, clickOptions);
            startMC.buttonMode = true;
            optionsMC.buttonMode = true;
        }
        
        public function clickPlay(e:MouseEvent)
        {
            dispatchEvent(new Event( PLAY ));
        }
        public function clickOptions(e:MouseEvent)
        {
            dispatchEvent(new Event( GAMEOPTIONS ));
        }
    }

}

GameScreen(this is where the problem is most likley):

package  
{
    import flash.display.MovieClip;
    import flash.ui.Keyboard;
    import flash.ui.Mouse;
    import flash.events.KeyboardEvent;
    import flash.events.Event;
    import flash.events.MouseEvent;
    
    public class GameScreen extends MovieClip
    { 
        public static const PAUSE:String = "pause";
        public static const WIN:String = "win";
        public static const LOSE:String = "lose";
        
        public var rightKey:Boolean = false;
        public var leftKey:Boolean = false;
        
        public var movementX:int = 25;
        
        public function GameScreen() 
        {
            trace("GameScreen")
            stage.addEventListener( KeyboardEvent.KEY_DOWN, keyPressed );
            stage.addEventListener( KeyboardEvent.KEY_UP, keyReleased );
            stage.addEventListener( Event.ENTER_FRAME, animate);
            stage.addEventListener( MouseEvent.MOUSE_MOVE, crosshair );
            //addEventListener( Event.ENTER_FRAME, collisionCheck);
        }
        
        public function keyPressed (e:KeyboardEvent):void
        {
            switch (e.keyCode)
            {
                case Keyboard.LEFT:
                    leftKey = true;
                    break;
                case Keyboard.RIGHT:
                    rightKey = true;
                    break;
            }

        }
        
        public function keyReleased (e:KeyboardEvent):void
        {
            switch (e.keyCode)
            {
                case Keyboard.LEFT:
                    leftKey = false;
                    break;
                case Keyboard.RIGHT:
                    rightKey = false;
                    break;
            }
        }
        
        public function animate (e:Event)
        {
            if ( leftKey )
            {
                ship_mc.x += -movementX;
            }
            if ( rightKey )
            {
                ship_mc.x += movementX;
            }
        }
        public function crosshair (e:Event):void
        {
            //mcCrosshair.gotoAndPlay(1);
            Mouse.hide();
            mcCrosshair.x = mouseX;
            mcCrosshair.y = mouseY;
        }
    }

}

I was told that this was being caused due to the fact that I was calling stage from a class other than my Document class, meaning that it didn’t know what stage was. That’s understandable, but what else could I possibly do to get this to work?

If I just have addEventListener and no stage. or if I have this.addEventListener, there will be no errors, but there will also be no movement.

Any help is appreciated as I’m still new at this.

Thanks.