TypeError: Error #1010 in AS3 constructor code

Question

I keep getting the TypeError: Error #1010 when trying to create the constructor code for a small game in AS3. The code that appears to be causing the issue is:

package  {
import flash.display.MovieClip;
import flash.events.MouseEvent;

public class Main extends MovieClip {
    var screen1:StartScreen;
    var screen2:InstructionsScreen;
    var screen3:SelectScreen;
    var screen4:Game1Screen;
    var screen5:Game2Screen;
    var screen6:Game3Screen;
    var screen7:FailScreen;
    var screen8:CompleteScreen;

    public function Main(){
        screen1 = new StartScreen();
        screen2 = new InstructionsScreen();
        screen3 = new SelectScreen();
        screen4 = new Game1Screen();
        screen5 = new Game2Screen();
        screen6 = new Game3Screen();
        screen7 = new FailScreen();
        screen8 = new CompleteScreen();

        screen1.startBtn.addEventListener(MouseEvent.CLICK,gotoSelect);
        screen1.instBtn.addEventListener(MouseEvent.CLICK,gotoInst);
        screen2.startBtn.addEventListener(MouseEvent.CLICK,gotoSelect2);
        screen3.game1Btn.addEventListener(MouseEvent.CLICK,gotoGame1);
        screen3.game2Btn.addEventListener(MouseEvent.CLICK,gotoGame2);
        screen3.game3Btn.addEventListener(MouseEvent.CLICK,gotoGame3);
        screen4.failBtn.addEventListener(MouseEvent.CLICK,gotoFail1);
        screen4.winBtn.addEventListener(MouseEvent.CLICK,gotoWin1);
        screen5.failBtn.addEventListener(MouseEvent.CLICK,gotoFail2);
        screen5.winBtn.addEventListener(MouseEvent.CLICK,gotoWin2);
        screen6.failBtn.addEventListener(MouseEvent.CLICK,gotoFail3);
        screen6.winBtn.addEventListener(MouseEvent.CLICK,gotoWin3);
        addChild(screen1);
    }
    private function gotoSelect(evt:MouseEvent):void{
        removeChild(screen1);
        addChild(screen3);
    }
    private function gotoInst(evt:MouseEvent):void{
        removeChild(screen1);
        addChild(screen2);
    }
    private function gotoSelect2(evt:MouseEvent):void{
        removeChild(screen2);
        addChild(screen3);
    }
    private function gotoGame1(evt:MouseEvent):void{
        removeChild(screen3);
        addChild(screen4);
    }
    private function gotoGame2(evt:MouseEvent):void{
        removeChild(screen3);
        addChild(screen5);
    }
    private function gotoGame3(evt:MouseEvent):void{
        removeChild(screen3);
        addChild(screen6);
    }
    private function gotoFail1(evt:MouseEvent):void{
        removeChild(screen4);
        addChild(screen7);
    }
    private function gotoWin1(evt:MouseEvent):void{
        removeChild(screen4);
        addChild(screen8);
    }
    private function gotoFail2(evt:MouseEvent):void{
        removeChild(screen5);
        addChild(screen7);
    }
    private function gotoWin2(evt:MouseEvent):void{
        removeChild(screen5);
        addChild(screen8);
    }
    private function gotoFail3(evt:MouseEvent):void{
        removeChild(screen6);
        addChild(screen7);
    }
    private function gotoWin3(evt:MouseEvent):void{
        removeChild(screen6);
        addChild(screen8);
    }
}
}

And the error message that appears when I try and run this is:

TypeError: Error #1010: A term is undefined and has no properties.
at Main()

Answer 1

I don't see anything wrong with the code itself. I'm guessing that one of the movieclips screen1 to screen6 don't have the named buttons defined, or the StartScreen etc classes don't exist or are accessible.