Small fix on expansion tile with json data

Question

I have a complex json. Using it must create expansion tiles. Imagine that in categories there are subcategories. In subcategories also have sub-sub categories. But model is same. I created that, but in Entry.fromJson() method need a small fix on children. I can't get child subcategories of categories. Please not use any plugin like flutter_treeview . Please help. Sample json string is in below:

[{"id":1,"name":"parent 1","subcategories":[{"id":11,"name":"cat news 1 of 1","subcategories":[]}]},{"id":2,"name":"parent 2","subcategories":[{"id":22,"name":"cat news 1 of 2","subcategories":[]}]},{"id":3,"name":"parent 3","subcategories":[]}]

The flutter code shown below:

class SubcategoryPage2 extends StatefulWidget {
  const SubcategoryPage2({Key? key}) : super(key: key);

  @override
  _SubcategoryPage2State createState() => _SubcategoryPage2State();
}

class _SubcategoryPage2State extends State<SubcategoryPage2> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('2nd test')),
      body: ListView.builder(
        itemCount: jsonDecode(sampleJson)[0].length,
        itemBuilder: (context, int index) =>
            EntryItem(Entry.fromJson(jsonDecode(sampleJson)[index])),
      ),
    );
  }
}

class Entry {
  final int id;
  final String name;
  final List<Entry> children;
  Entry(
      {required this.id, required this.name, this.children = const <Entry>[]});

  factory Entry.fromJson(Map<String, dynamic> json) {
    return Entry(
        id: json['id'],
        name: json['name'],
        children: List<Entry>.from(json['subcategories'])
        );
  }
}
class EntryItem extends StatelessWidget {
  final Entry entry;
  const EntryItem(this.entry, {Key? key}) : super(key: key);

  Widget _buildTiles(Entry root) {
    if (root.children.isEmpty) {
      return ListTile(title: Text(root.name));
    }
    return ExpansionTile(
      key: PageStorageKey<Entry>(root),
      title: Text(root.name),
      children: root.children.map(_buildTiles).toList(),
    );
  }

  @override
  Widget build(BuildContext context) {
    return _buildTiles(entry);
  }
}