message: Add `null` as valid type for DisplayObject's mask
diff: @@ -221,7 +221,7 @@ export abstract class DisplayObject extends EventEmitter
  /**
  * The original, cached mask of the object.
  *
  - * @member {PIXI.Graphics|PIXI.Sprite|null}
  + * @member {PIXI.Container|PIXI.MaskData|null}
  * @protected
  */
  this._mask = null;
  @@ -775,9 +775,9 @@ export abstract class DisplayObject extends EventEmitter
  * sprite.mask = graphics;
  * @todo At the moment, PIXI.CanvasRenderer doesn't support PIXI.Sprite as mask.
  *
  - * @member {PIXI.Container|PIXI.MaskData}
  + * @member {PIXI.Container|PIXI.MaskData|null}
  */
  - get mask(): Container|MaskData
  + get mask(): Container|MaskData|null
  {
  return this._mask;
  }
  