parser(value) {
    if (
      this.requireOptionMatch &&
      this.checkedIndex === -1 &&
      value !== '' &&
      !Array.isArray(value)
    ) {
      return new Unparseable(value);
    }
    return super.parser(value);
  }
Converts viewValue to modelValue
@override CustomChoiceGroupMixin
@param {string|string[]} value - viewValue: the formatted value inside <input>
@returns {*} modelValue