Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / modules / quickedit / js / models / AppModel.js

/**
* DO NOT EDIT THIS FILE.
* See the following change record for more information,
* https://www.drupal.org/node/2815083
* @preserve
**/

(function (Backbone, Drupal) {
  Drupal.quickedit.AppModel = Backbone.Model.extend({
    defaults: {
      highlightedField: null,

      activeField: null,

      activeModal: null
    }
  });
})(Backbone, Drupal);