bool _shouldCollapse = true;

onTreeReady: (controller) {
    if (_shouldCollapse && rootNode.children.isNotEmpty) {
      WidgetsBinding.instance.addPostFrameCallback((_) {
        if (mounted) {
            controller.collapseNode(rootNode.children.first as IndexedTreeNode<NodePayload>);
            setState(() => _shouldCollapse = false);
        }
      });
    }
},