removeAt method
Implementation
ITreeNode<Data> removeAt(int index) {
if (index < 0 || index > _flatList.length)
throw RangeError.index(index, _flatList);
_itemsMap.remove(_flatList[index].path);
final removedItem = _flatList.removeAt(index);
_listState.removeItem(index, removedItem);
return removedItem;
}