[]Cannot read property 'load' of null

查了断点，看到Laya.loader是null，为什么会出现为null的情况？ 
代码如下:

class main{
constructor(){
MapManager.getInstance().LoadMapResource();
}
}
new main();


class MapManager
{
private static s_instance = null;
public static getInstance():MapManager
{
if(null==this.s_instance)
{
this.s_instance=new MapManager();
this.s_instance.init();
}
return this.s_instance;
}
private init()
{
}
private skins: Array<string>;
private mapImg:Laya.Image;
private constructor()
{
}
public GetMap():Laya.Image
{
if(this.mapImg==null)
{
this.mapImg=new Laya.Image();
}
return this.mapImg;
}
public LoadMapResource() {
this.skins=[
"res/img/map/1.jpg",
"res/img/map/2.jpg",
"res/img/map/3.jpg",
"res/img/map/4.jpg",
"res/img/map/5.jpg",
"res/img/map/6.jpg",
"res/img/map/7.jpg",
"res/img/map/8.jpg",
"res/img/map/9.jpg",
"res/img/map/10.jpg",
"res/img/map/12.jpg",
"res/img/map/13.jpg",
"res/img/map/14.jpg",
"res/img/map/15.jpg",
"res/img/map/16.jpg",
"res/img/map/17.jpg",
"res/img/map/18.jpg",
"res/img/map/19.jpg",
"res/img/map/20.jpg",
"res/img/map/21.jpg",
"res/img/map/22.jpg",
"res/img/map/23.jpg",
"res/img/map/24.jpg",
"res/img/map/25.jpg",
"res/img/map/26.jpg",
"res/img/map/27.jpg",
"res/img/map/28.jpg",
"res/img/map/29.jpg",
];
Laya.loader.load(this.skins,laya.utils.Handler.create(this, this.LoadSuccess,null,false));
}
private LoadSuccess()
{
ViewManager.getInstance().CreateView(contant.loginId,new LoginView());
ViewManager.getInstance().ShowView(contant.loginId);
}
public SwictMap(index: number):void
{
var mapName="res/img/map/"+index;
this.mapImg.loadImage(mapName);
}
}