removeItemAt method
Implementation
void removeItemAt(int index) {
  super.onNotifyWithCallback(() {
    var list = List<T>.from(items);
    list.removeAt(index);
    items = list;
  });
}
void removeItemAt(int index) {
  super.onNotifyWithCallback(() {
    var list = List<T>.from(items);
    list.removeAt(index);
    items = list;
  });
}