expandNode method
Implementation
void expandNode(ITreeNode<Data> item) {
if (item.childrenAsList.isEmpty || item.isExpanded) return;
animatedListStateController.insertAll(
animatedListStateController.indexOf(item) + 1,
List.from(item.childrenAsList),
);
item.expansionNotifier.value = true;
item.cacheChildIndices();
}