[LayaAir 2.0]Laya.timer.loop() 报错 Cannot read property 'loop' of null

import { Image } from "laya/ui/Image";

// import { List } from "laya/ui/List";

import { Laya } from "laya";



export default class test_move extends Image{

    // /** @prop {name:move_img, tips:"用来移动的箭头", type:Image} */

    // public _move_img: Image;

    // public _list: List; //创建一个 List 类的实例对象 list 。

    private n:number = 0;

    private x0:number = 0;

    private y0:number = 0;



    constructor(){super();}



    onEnable():void{

        this.x = this.x0;

        this.y = this.y0;

        Laya.timer.loop(20, this, this.frameWork); // 每 20 ms 执行一次

    }



    private frameWork():void{

        this.y = this.y0 + 10 * (this.n % 3);

        alert("move ");

    }



}

Laya.timer.loop(20, this, this.frameWork);