Question

What am i doing wrong?
the startButton is a button and inside a movieclip (menuScreen)
the instancename of the button is startButton.

Line 12 1120: Access of undefined property startButton.
Line 13 1120: Access of undefined property startButton

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

public class MenuScreen extends MovieClip 
{
    var startButton:StartButton;
    public function MenuScreen();
    {
        startButton = new StartButton();
        startButton.addEventListener( MouseEvent.CLICK, onClickStart );
    }

    public function onClickStart( event:MouseEvent ):void
    {
        dispatchEvent( new NavigationEvent( NavigationEvent.START ) );
    }
}
}

Answer 1

You’ve got an extra semicolon on this line:

public function MenuScreen();

which is making Flash think the following {} block is unrelated to MenuScreen().

Also, as Johan points out, having a StartButton instance on the timeline means that lines 9 and 12 are essentially already done for you, so you don’t need to declare the variable or create an instance.