pageLabelToPageNumber(label) {
    if (!this._pageLabels) {
      return null;
    }
    const i = this._pageLabels.indexOf(label);
    if (i < 0) {
      return null;
    }
    return i + 1;
  }
@param {string} label - The page label.
@returns {number|null} The page number corresponding to the page label,
  or `null` when no page labels exist and/or the input is invalid.