调用按钮时连续获取类型错误#1010

这是代码

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

public class Main extends MovieClip
{
    var startPage:StartPage;
    var hillPage:HillPage;
    var pondPage:PondPage;

    public function Main()
    {
        startPage = new StartPage;
        hillPage = new HillPage;
        pondPage = new PondPage;
        addChild(startPage);

        //Add event listeners
        startPage.hillButton.addEventListener(MouseEvent.CLICK, onHillButtonClick);
        startPage.pondButton.addEventListener(MouseEvent.CLICK, onPondButtonClick);
        hillPage.backToStartButton.addEventListener(MouseEvent.CLICK, onBackButtonClick_Hill);
        pondPage.backToStartButton.addEventListener(MouseEvent.CLICK, onBackButtonClick_Pond);
    }
    //Event handlers
    function onHillButtonClick(event:MouseEvent):void
    {
        addChild(hillPage);
        removeChild(startPage);
    }
    function onPondButtonClick(event:MouseEvent):void
    {
        addChild(pondPage);
        removeChild(startPage);
    }
    function onBackButtonClick_Hill(event:MouseEvent):void
    {
        addChild(startPage);
        removeChild(hillPage);
    }
    function onBackButtonClick_Pond(event:MouseEvent):void
    {
        addChild(startPage);
        removeChild(pondPage);
    }
}
}

所以我可以很好地访问HillButton和PondButton,但backToStartButton似乎在向我抛出未定义的术语。如果有任何帮助,我们将不胜感激

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