[LayaAir 1.0]使用mask后，只要调用 Laya.Resource.destroyUnusedResources() 就会引起报错

重现demo如下：

Laya.init(600, 400, WebGL);
let bgSprite = new Laya.Sprite();
bgSprite.graphics.drawRect(0, 0, 200, 200, "#0xFFFF00");
bgSprite.pos(300, 300);
Laya.stage.addChild(bgSprite);

let mask = new Laya.Sprite();
bgSprite.mask = mask;
mask.graphics.drawCircle(0, 0, 100, "0x0000FF");

window.onblur = function()
{
console.log("onblur....?");
Laya.timer.once(2000, null, function()
{
Laya.Resource.destroyUnusedResources();
});
}


当触发了释放资源的函数，会报错如下：
laya.webgl.js:5713
Uncaught TypeError: Cannot read property '_targets' of null
at WebGLContext2D.__proto.drawCanvas (laya.webgl.js:5713)
at RenderContext.__proto.drawCanvas (laya.core.js:5822)
at RenderSprite3D.__proto._canvas (laya.core.js:6219)
at Sprite.__proto.render (laya.core.js:15680)
at RenderSprite3D.__proto._childs (laya.core.js:6201)
at RenderSprite3D.__proto._transform (laya.webgl.js:6758)
at Stage.__proto.render (laya.core.js:15680)
at Stage.__proto.render (laya.core.js:18616)
at Stage.__proto._loop (laya.core.js:18546)
at loop (laya.core.js:5449)

对于这个问题的出现表示不解，为什么mask会被释放了？即使mask是loadImage加载图片来使用，也是一样有这个问题