FireBase Auth createUserWithEmailAndPassword().then() not working

I am working on an android application that involves user firebase auth for user sign up. The sign up works but I want to add username to database by implementing it in .then() but my android studio keeps giving "unresolved method then(?)". I am also using catch but that seems to work fine.The code I am writing is as following where firebaseAuth is an object of type FirebaseAuth:

            firebaseAuth.createUserWithEmailAndPassword(email,password)
                .then( (u) => {
                        //Implementation of then
                })
                .catch(error => {
                switch (error.code) {
                    case 'auth/email-already-in-use':
                        EmailWarning.setText("Email already in use");
                    case 'auth/invalid-email':
                        EmailWarning.setText("Invalid Email");
                    case 'auth/weak-password':
                        PasswordWarning.setText("Password should be 8 characters or longer");
                    default:
                        PasswordWarning.setText("Error during sign up");
                }
        });

I found a similar problem in following link but even after trying this, it's not working.

I looked further into the firebase documentation and found another implementation which uses on complete listener here however the error codes described in this documentation doesn't seem to work with it.

Update 1: I ended up implementing on complete listener as following:

            firebaseAuth.createUserWithEmailAndPassword(email,password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            // Sign in success, update UI with the signed-in user's information
                            FirebaseUser user = firebaseAuth.getCurrentUser();
                            UserProfileChangeRequest profileUpdates = new UserProfileChangeRequest.Builder()
                                    .setDisplayName(Username)
                                    .build();

                            user.updateProfile(profileUpdates);
                        }
                        else {
                            // If sign up fails, display a message to the user.
                            switch (task.getException()) {
                                case "auth/email-already-in-use":
                                    EmailWarning.setText("Email already in use");
                                case "auth/invalid-email":
                                    EmailWarning.setText("Invalid Email");
                                case "auth/weak-password":
                                    PasswordWarning.setText("Password should be 8 characters or longer");
                                default:
                                    PasswordWarning.setText("Error during sign up");
                            }
                        }

                    }
                });

Now the user adding part works fine but the exception handler doesn't work with strings so I can't find a way to work with error codes given on firebase documentation