addChoice method
Implementation
List<MultiselectOption> addChoice(
MultiselectOption choice, bool? multiSelect) {
List<MultiselectOption> newValues = multiSelect == true ? [...state] : [];
// Remove the choice if it already exists.
if (newValues.firstWhereOrNull(
(MultiselectOption option) => option.value == choice.value) !=
null) {
newValues = newValues.where((val) => val.value != choice.value).toList();
} else {
newValues.add(choice);
}
state = newValues;
return newValues;
}