按钮在flash actionscript 3.0 TypeError＃1009中不起作用

Question

我正在构建一个简单的计算器但是当我单击包含数字的按钮时，它将不会显示在我推出的动态文本框中。我已经检查了我的实例名称，重新检查代码，他们都很好，我似乎得到了错误TypeError: Error #1009: Cannot access a property or method of a null object reference. at game/frame1()

        import flash.events.MouseEvent;

    one_btn.addEventListener(MouseEvent.CLICK, displayText);
    two_btn.addEventListener(MouseEvent.CLICK, displayText);
    three_btn.addEventListener(MouseEvent.CLICK, displayText);

function displayText(event:MouseEvent):void {
    trace(event.target.name);

    if(event.target.name == "one_btn"){
        txtAnswer.appendText("1");
    }

    if(event.target.name == "two_btn"){
        txtAnswer.appendText("2");
    }

    if(event.target.name == "three_btn"){
        txtAnswer.appendText("3");
    }
}