Error "Null check operator used on a null value" in flutter

error

I tried to fetch user’s data from firebase using logged user uid . And also I used model

model code

Details detailsFromJson(String str) => Details.fromJson(json.decode(str));

class Details {
  Details({
    required this.id,
    required this.age,
    required this.drinkstalkSE,
    required this.drinksUnderstandSE,
    required this.familyChildren,
   
  });
  String id;
  String age;
  String drinkstalkSE;
  String drinksUnderstandSE;
  String familyChildren;
 

  factory Details.fromJson(Map<String, dynamic> json) => Details(
        id: json["id"] ?? "",
        age: json["age"] ?? "",
        drinkstalkSE: json["drinkstalkSE"] ?? "",
        drinksUnderstandSE: json["drinksUnderstandSE"] ?? "",
        familyChildren: json["familyChildren"] ?? "",
        
      );

  Map<String, dynamic> toJson() => {
        "id": id,
        "age": age,
        "drinkstalkSE": drinkstalkSE,
        "drinksUnderstandSE": drinksUnderstandSE,
        "familyChildren": familyChildren,
       
      };
}

backend code

 bool loading = false;

  @override
  initState() {
    super.initState();
    loading = true;

    getDetails();
  }


  Details? oneUserDetails;
  Future<void> getDetails() async {
    final sp = context.read<SignInProvider>();
    final id = sp.uid;
    final reference = FirebaseFirestore.instance.doc('users/$id');
    final snapshot = reference.get();

    final result = await snapshot.then(
        (snap) => snap.data() == null ? null : Details.fromJson(snap.data()!));
    print('result is ====> $result');
    setState(() {
      oneUserDetails = result;
      loading = false;
    });
  }

screen code

Database image

In my code no any errors show I think problem have in my model but I couldn’t understand what is the error and how to solve it. How to solve this error?