How to write exception in build of AsyncNotifier from riverpod - flutter

Question

When not connected to the internet, executing the following code will cause a _ClientSocketException.
How should I write the exception handling?
class AsyncTodosNotifier extends AsyncNotifier<List<Todo>> {
Future<List<Todo>> _fetchTodo() async {
final json = await http.get('api/todos'); //** _ClientSocketException Error occurred**
final todos = jsonDecode(json) as List<Map<String, dynamic>>;
return todos.map((todo) => Todo.fromJson(todo)).toList();
}
#override
Future<List<Todo>> build() async {
return _fetchTodo();
}
Using the AsyncNotifier's build() from riverpod, I would like to code exception handling with AsyncValue.guard, but it results in a syntax error.
How should I write it to make it work?
When trying to get json data, if I can't connect to the internet, I want to write exception handling so that it doesn't abort.
Reference:
https://docs-v2.riverpod.dev/docs/providers/notifier_provider
full code:
implementation.
#immutable
class Todo {
const Todo({
required this.id,
required this.description,
required this.completed,
});
factory Todo.fromJson(Map<String, dynamic> map) {
return Todo(
id: map['id'] as String,
description: map['description'] as String,
completed: map['completed'] as bool,
);
}
final String id;
final String description;
final bool completed;
Map<String, dynamic> toJson() => <String, dynamic>{
'id': id,
'description': description,
'completed': completed,
};
}
class AsyncTodosNotifier extends AsyncNotifier<List<Todo>> {
Future<List<Todo>> _fetchTodo() async {
final json = await http.get('api/todos');　//** _ClientSocketException Error occurred**
final todos = jsonDecode(json) as List<Map<String, dynamic>>;
return todos.map((todo) => Todo.fromJson(todo)).toList();
}
#override
Future<List<Todo>> build() async {
return _fetchTodo();
}
Future<void> addTodo(Todo todo) async {
state = const AsyncValue.loading();
state = await AsyncValue.guard(() async {
await http.post('api/todos', todo.toJson());
return _fetchTodo();
});
}
}
final asyncTodosProvider =
AsyncNotifierProvider<AsyncTodosNotifier, List<Todo>>(() {
return AsyncTodosNotifier();
});