[LayaAir 2.0]Sprite 设置 cacheAs = "bitmap"报错

版本：2.0.0 beta5.1
TypeError: Cannot read property 'context' of nullpreload.js:55
    at WebGLContext2D.__proto.drawCanvas (/***/bin/libs/laya.webgl.js:12261:18)
    at RenderSprite3D.__proto._canvas (/***/bin/libs/laya.core.js:8717:12)
    at RenderSprite3D.RenderSprite._initRenderFun [as _fun] (/***/bin/libs/laya.core.js:8810:6)
    at Box.__proto.render (/***/bin/libs/laya.core.js:18283:43)
    at RenderSprite3D.__proto._children (/***/bin/libs/laya.core.js:8676:40)
    at IsoMap.__proto.render (/***/bin/libs/laya.core.js:18283:43)
    at RenderSprite3D.__proto._children (/***/bin/libs/laya.core.js:8676:40)
    at Sprite.__proto.render (/***/bin/libs/laya.core.js:18283:43)
    at RenderSprite3D.__proto._children (/***/bin/libs/laya.core.js:8676:40)
    at Sprite.__proto.render (/***/bin/libs/laya.core.js:18283:43)