async getXfa() {
    return this._transport._htmlForXfa?.children[this._pageIndex] || null;
  }
@returns {Promise<Object | null>} A promise that is resolved with
  an {Object} with a fake DOM object (a tree structure where elements
  are {Object} with a name, attributes (class, style, ...), value and
  children, very similar to a HTML DOM tree), or `null` if no XFA exists.