message: Fix for the mask type declaration
diff: @@ -152,7 +152,7 @@ export default class DisplayObject extends EventEmitter
  /**
  * The original, cached mask of the object.
  *
  - * @member {PIXI.Graphics|PIXI.Sprite}
  + * @member {PIXI.Graphics|PIXI.Sprite|null}
  * @protected
  */
  this._mask = null;
  @@ -680,7 +680,7 @@ export default class DisplayObject extends EventEmitter
  * sprite.mask = graphics;
  * @todo At the moment, PIXI.CanvasRenderer doesn't support PIXI.Sprite as mask.
  *
  - * @member {PIXI.Graphics|PIXI.Sprite}
  + * @member {PIXI.Graphics|PIXI.Sprite|null}
  */
  get mask()
  {
  