copyWith method Null safety
inherited
    Implementation
User copyWith(
    {String? id,
    String? firstname,
    String? lastname,
    String? email,
    String? password}) {
  return User(
    id: id ?? this.id,
    firstname: firstname ?? this.firstname,
    lastname: lastname ?? this.lastname,
    email: email ?? this.email,
    password: password ?? this.password,
  );
}