addFormElement(child, indexToInsertAt) {
    super.addFormElement(child, indexToInsertAt);
    // the first elements checked by default
    if (
      !this.hasNoDefaultSelected &&
      !this.__hasInitialSelectedFormElement &&
      (!child.disabled || this.disabled)
    ) {
      /* eslint-disable no-param-reassign */
      child.active = true;
      child.checked = true;
      /* eslint-enable no-param-reassign */
      this.__hasInitialSelectedFormElement = true;
    }
    this._alignInvokerWidth();

    this._onFormElementsChanged();
  }
@enhance FprmRegistrarMixin make sure children have specific default states when added
@param {LionOption & FormControlHost} child
@param {Number} indexToInsertAt