am tying to calling user to appear in profile page the app block when i fixed it i faced other problem which is this one if there any solution can help me

```dart
import 'package:flutter/foundation.dart';

class UserModel {
  String? uid;
  String? Username;
  String? email;
  String? photoUrl;

  UserModel(
      {this.uid, this.email, this.Username,  this.photoUrl});

  // receving data from the server
  factory UserModel.fromMap(Map) {
    return UserModel(
      uid: Map['userId'],
      Username: Map['Username'],
      email: Map['email'],
      photoUrl: Map['photoUrl'],
    );
  }

  // /// sending data to firestore
  Map<String, dynamic> toMap() {
    return {
      'userId': uid,
      'Username': Username,
      'email': email,
      'photoUrl': photoUrl,
    };
  }
}

```

**the error picture**
[![enter image description here](https://i.stack.imgur.com/nWqAZ.png)](https://i.stack.imgur.com/nWqAZ.png)