I have a code which is correct in a 3 years back flutter but not in the current flutter version

This is a class UniqueId which is a valueobject. This is for checking equality and all that

class UniqueId extends ValueObject<String> {
      @override
      final Either<ValueFailure<String>, String> value;
    
      factory UniqueId() {
        return UniqueId._(
          Right(const Uuid().v1())
        );
      }
      factory UniqueId.fromUniqueString(String uniqueId) {
        return UniqueId._(Right(uniqueId));
      }
    
     const UniqueId._(
        this.value,
      );
    }

Then I have used freezed.

@freezed
abstract class User1 with _$User1{
  const factory User1(
    {required UniqueId id,}
  ) = _User1;
}

Now I am giving the part I am having problem with

@LazySingleton(as: IauthFailure)
class FirebaseAuthFailure implements IauthFailure {
  final FirebaseAuth firebaseAuth;
  final GoogleSignIn googleSignIn;

  FirebaseAuthFailure(this.firebaseAuth, this.googleSignIn); 
  @override
  Future<Option<User1>> getSignedInUser() =>
      firebaseAuth.currentUser().then((firebaseUser) => 
      optionOf(firebaseUser?.toDomain()));

The error I am getting is given below:
error1: The function can't be unconditionally invoked because it can be 'null'.
solution I used:

 @override
  Future<Option<User1>> getSignedInUser() =>
      firebaseAuth.currentUser!().then((firebaseUser) => optionOf(firebaseUser?.toDomain()));

error2:the expression doesn't evaluate to a function, so it can't be invoked.
solution I used:

 @override
  Future<Option<User1>> getSignedInUser() =>
      firebaseAuth.currentUser!.then((firebaseUser) => optionOf(firebaseUser?.toDomain()));

error3:The method 'then' isn't defined for the type 'User'.
Try correcting the name to the name of an existing method, or defining a method named 'then'.
solution I used: Cannot find a solution
The definition of toDomain() is :

extension FirebaseUserDomainX on User {
  User1 toDomain() {
    return User1(id: UniqueId.fromUniqueString(uid));
  }
}

Actually I am not getting this toDomain when I am hovering over it and click go to definition its saying no defintion.

If i understood it correctly (your question is not very clear), you should update your code accordingly to deprecated methods.
currentUser is not a method anymore, but it is a getter.
also I'm assuming _firebaseAuth is an instance of FirebaseAuth

@override
Future<Option<User>> getSignedInUser() async {
  User? firebaseUser = _firebaseAuth.currentUser;
  return optionOf(firebaseUser?.toDomain());
}

It would be very valuable if you posted the error that you're getting when using optionOf

I have found the solution of this error this code is deprecated. that's why is not working properly.

@override
  Future<Option<User1>> getSignedInUser() =>
      firebaseAuth.currentUser!.then((firebaseUser) => optionOf(firebaseUser?.toDomain()));

see this solution and try it. I have made a simple solution for the deprecated code it's run properly.

  @override
  Future<Option<User>> getSignedInUser() async {
    final firebaseUser = _firebaseAuth.currentUser;

    if (firebaseUser != null) {
      return optionOf(firebaseUser.toDomain());
    } else {
      return none();
    }
  }