insertItem method Null safety
Insert an item at index
and start an animation that will be passed
to AnimatedList.itemBuilder when the item is visible.
This method's semantics are the same as Dart's List.insert method:
it increases the length of the list by one and shifts all items at or
after index
towards the end of the list.
Implementation
void insertItem(int index, { Duration duration = _kDuration }) {
_sliverAnimatedListKey.currentState!.insertItem(index, duration: duration);
}