getRequiredWidgetUrl(mode = undefined) {
    const url = /** @type {string} */ (this.config_.getRequired('widgetUrl'));
    return this.widgetUrlForMode_(url, mode);
  }
Gets the widget URL for a specific mode.
The 'widgetUrl' configuration is required for this method.
@param {?Config.WidgetMode=} mode The mode for the widget.
@return {string} The URL of the callback widget.