[LayaAirIDE 2.0]加载UI为什么总是加载白底

var LayaSample = (function(){
(function(){
//初始化引擎

Laya.init(750,1334);
//设置stage颜色s
Laya.stage.bgColor = "#ffffff";
//加载资源
Laya.loader.load("res/atlas/comp.json",Laya.Handler.create(this,onLoaded),null,Laya.Loader.ATLAS)
})();
function onLoaded(){
var _start = new start();
Laya.stage.addChild(_start);

}
})();


var start = (function(_super){
function start(){
start.super(this);

}
Laya.class(start,"start",_super);
//var _proto = Game.prototype;


return start;

})(ui.startUI)