[]官方视频教程中飞机大战 "this.addChild is not a function"

IDE Layabox1.7.20.2 beta 版
src/Game.js

//var WebGL = laya.webgl.WebGL;

//Laya.init(480, 852, WebGL);

var Game = (function(){

    (function Game(){

        Laya.init(480,852);

        this.bg = BackGround();

        Laya.stage.addChild(this.bg);

    })();

})();

src/BackGround.js

var BackGround = (function(_super){

    function BackGround(){

        BackGround.super(this);

        this.bg1 = new Laya.Sprite();

        this.bg1.loadImage("war/background.png");

        this.addChild(this.bg1);

    }

    Laya.class(BackGround,"BackGround",_super);

    return BackGround;

})(Laya.Sprite);

报错
"TypeError: this.addChild is not a function    at BackGround (file:///D:/test/laya/myLaya/src/BackGround.js:6:14)    at Game (file:///D:/test/laya/myLaya/src/Game.js:6:19)    at file:///D:/test/laya/myLaya/src/Game.js:8:7    at file:///D:/test/laya/myLaya/src/Game.js:9:3"