Flutter – The argument type error: String cant be assigned to int in firebase function

I am creating a sign up and sign in with email and password funcion using firebase. I have a user model in which I add or update to the firestore when user has not been registered or sign in. but I get an error from the function saying The argument type ‘String’ can’t be assigned to the parameter type ‘int’.
This is my User Model:

import 'package:cloud_firestore/cloud_firestore.dart';

class UserModel {
  late final int uid;
  late final String email;
  late final String displayName;
  late final String photoUrl;
  late final bool isAdmin;
  late final bool isGoogleUser;

  UserModel(
      {required this.uid,
      required this.email,
      required this.displayName,
      required this.photoUrl,
      this.isAdmin = false,
      this.isGoogleUser = false});

  factory UserModel.fromFirestore(DocumentSnapshot snapshot) {
    return UserModel(
      uid: snapshot['uid'],
      email: snapshot['email'],
      displayName: snapshot['displayName'],
      photoUrl: snapshot['photoUrl'],
      isAdmin: snapshot['isAdmin'] ?? false,
      isGoogleUser: snapshot['isGoogleUser'] ?? false,
    );
  }
}

This is the register with email and password function as well as the sign in with email and password function;

//Registering a new user unto firebase....................................................
  Future<AuthStatus> registerWithEmailAndPassword(
      String username, String email, String password) async {
    try {
      final UserCredential userCredential = await _auth
          .createUserWithEmailAndPassword(email: email, password: password);

      if (userCredential.user != null) {
        UserModel userModel = UserModel(
            uid: userCredential.user!.uid, //error: The argument type 'String' can't be assigned to the parameter type 'int'.
            email: email,
            displayName: username,
            photoUrl: '');

        // Add the UserModel data to a Firestore collection
        await _usersRef.doc(userModel.uid).set({ //error: The argument type 'int' can't be assigned to the parameter type 'String?'.
          'uid': userModel.uid,
          'email': userModel.email,
          'username': userModel.displayName,
          'photoUrl': userModel.photoUrl,
        });

        // Return a successful AuthStatus
        return AuthStatus(
          isSuccessful: true,
          errorMessage: '',
          userModel: userModel,
        );
      } else {
        // If userCredential.user is null, registration failed
        return AuthStatus(
          isSuccessful: false,
          errorMessage: 'User registration failed',
          userModel:
              UserModel(uid: '', email: '', displayName: '', photoUrl: ''),//error: The argument type 'String' can't be assigned to the parameter type 'int'. 
        );
      }
    } catch (e) {
      // Handle any errors during registration
      return AuthStatus(
        isSuccessful: false,
        errorMessage: e.toString(),
        userModel: UserModel(uid: '', email: '', displayName: '', photoUrl: ''),//error: The argument type 'String' can't be assigned to the parameter type 'int'.
      );
    }
  }

  //SignIn a registered member or user to firebase.........................................

  Future<UserModel?> signInWithEmailAndPassword(
      String username, String password) async {
    try {
      final UserCredential userCredential =
          await _auth.signInWithEmailAndPassword(
        email: username,
        password: password,
      );

      if (userCredential.user != null) {
        UserModel userModel = UserModel(
          uid: userCredential.user!.uid, //error: The argument type 'String' can't be assigned to the parameter type 'int'.
          email: userCredential.user!.email ?? '',
          displayName: userCredential.user!.displayName ?? '',
          photoUrl: '',
        );

        return userModel;
      } else {
        return null;
      }
    } catch (e) {
      print('Sign-in error: $e');
      return null;
    }
  }

This is the AuthStatus class:

import 'package:kp_app/models/user_model.dart';

class AuthStatus {
  final bool isSuccessful;
  final String errorMessage;
  final UserModel userModel;

  AuthStatus({
    required this.isSuccessful,
    required this.errorMessage,
    required this.userModel,
  });
}