[LayaAirIDE 2.0]加载.ls场景文件，销毁场景，再加载同一场景，想恢复初始状态，结果报错【附Demo】

报错内容：
Uncaught TypeError: Cannot read property 'length' of null
    at Scene3D.__proto._activeHierarchy (laya.core.js:13721)
    at Scene3D.__proto._onAdded (laya.core.js:13770)
    at Scene3D.__proto._setParent (laya.core.js:13486)
    at Scene3D.__proto._setParent (laya.core.js:18511)
    at Stage.__proto.addChild (laya.core.js:13290)
    at Main.Complete3D (Main.ts:48)
    at Handler.__proto.runWith (laya.core.js:1400)
    at LoaderManager.__proto._createOne (laya.core.js:14594)
    at LoaderManager.__proto._create (laya.core.js:14560)
    at LoaderManager.__proto.create (laya.core.js:14514)