我在下面一行中的“snap.snapshot.value”参数上有一个错误“var map = Map〈String，dynamic〉.from（snap.snapshot.value）;“。错误为“无法将参数类型”Object？“赋给参数类型”Map〈dynamic，dynamic〉“。“

class _HomePageState extends State<HomePage> {

  List<Posts> postsList = [];

  @override
  void initState() {

    super.initState();

    DatabaseReference postsRef = FirebaseDatabase.instance.reference().child("Posts");

    postsRef.once().then((snap) {

      var map = Map<String, dynamic>.from(snap.snapshot.value); <--- the error is here

      postsList.clear();

      map.forEach((key, value) {
        var values = Map<String,dynamic>.from(map);
        Posts posts = Posts
          (
            values['url'],
            values['descrizione'],
            values['data'],
            values['ora']
        );
        postsList.add(posts);
      });