从实时firebase数据库接收数据时出现问题

这个错误是参数类型“Null Function(DataSnapshot)”不能分配给参数类型“FutureOr Function(DatabaseEvent)”

@override
  void initState() {
    super.initState();
    final fb =  FirebaseDatabase.instance;
    final postsRef = fb.reference();
    postsRef.child("Posts").once().then((DataSnapshot data)
    {
      var KEYS = data.value.keys;
      var DATA = data.value;
      postsList.clear();
      for (var individualKey in KEYS)
      {
        Posts posts = Posts
          (
          DATA[individualKey]['url'],
          DATA[individualKey]['descrizione'],
          DATA[individualKey]['data'],
          DATA[individualKey]['ora'],
        );
        postsList.add(posts);
      }
      setState(()
      {
        print('Length : $postsList.length');
      });
    });
  }

我解决不了这个问题，有人能帮我吗？