Null Pointer when trying to retrieve user details from Firebase Realtime Database

In my Android app, I have a database repository to contact Firebase Realtime Database and a service layer with some methods. My SaveUserProfile method works but I keep getting a null pointer on my GetUserFromUid.

The 'user' object it returns is null and I don't know why. The node in my DB is called "users" (all lowercase) and I want to retrieve a user as a model via their userId and display the name and email onscreen.

Can anybody see where I'm going wrong?

My DbContext:

public class DbContext implements IDbContext {

User user = null;
Context context;
DatabaseReference databaseUsers = FirebaseDatabase.getInstance().getReference("users");

 public DbContext(Context context){
   super();
   this.context = context;
}

@Override
public void AddUserAccount(User user1) {
    databaseUsers.child(user1.userId).setValue(user1);
}

@Override
public User GetUserFromFirebase(String uid) {
   databaseUsers.child(uid)
           .addListenerForSingleValueEvent(new ValueEventListener() {
               @Override
               public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                   user = dataSnapshot.getValue(User.class);
               }

               @Override
               public void onCancelled(@NonNull DatabaseError databaseError) {

               }
           });
    return user;
}

My DbService:

public class DbService implements IDbService {

//instance of DbContext for firebase handling
private DbContext dbContext;

public DbService(Context context){
    super();
    dbContext = new DbContext(context);
}

@Override
public User SaveUserProfile(User u) {

    dbContext.AddUserAccount(u);

    return u;
}

@Override
public User GetUserFromUid(String uid) {
    User user = dbContext.GetUserFromFirebase(uid);
    return user;
}

My User model:

public class User {

public String userId;
public String name;
public String email;
public String account;

//constructor required for calls to DataSnapshot.getValue(User.class)
public User(){
}

public User(String userId, String name, String email, String account) {
    this.userId = userId;
    this.name = name;
    this.email = email;
    this.account = account;
}

public String getUserId() {
    return userId;
}

public void setUserId(String userId) {
    this.userId = userId;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getAccount() {
    return account;
}

public void setAccount(String account) {
    this.account = account;
}

My activity where I want to display the users' details:

public class DetailsActivity extends AppCompatActivity {
//tag
private static final String TAG = DetailsActivity.class.getSimpleName();

//firebase auth
private FirebaseAuth mAuth;

//variables
private TextView inputName, inputEmail;
private DatabaseReference mFirebaseDatabase;
private String userId;
public String currentUserAccount;
public String teacherAccountNav = "Teacher";
public User userDetails;

@RequiresApi(api = Build.VERSION_CODES.KITKAT)
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_details);

    inputName = findViewById(R.id.nameTextView);
    inputEmail = findViewById(R.id.emailTextView);

   

    FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();

    assert user != null;
    userId = user.getUid();

   
    getUserDetails(userId);
    inputName.setText(userDetails.name);
    inputEmail.setText(userDetails.email);
}

public User getUserDetails(String uid){
    DbService dbService = new DbService(this);
    userDetails = dbService.GetUserFromUid(uid);
    return userDetails;
}

EDITS BELOW

My callback:

public interface Callback {
void myResponseCallback(User user);
}

My EDITED DbContext:

public class DbContext implements IDbContext {

User user = null;
Context context;
DatabaseReference databaseUsers = FirebaseDatabase.getInstance().getReference("users");

public DbContext(Context context) {
    this.context = context;
}

@Override
public void AddUserAccount(User user1) {
    databaseUsers.child(user1.userId).setValue(user1);
}

@Override
public void GetUserFromFirebase(String uid, final Callback callback) {
    databaseUsers.child(uid)
            .addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    user = dataSnapshot.getValue(User.class);
                    callback.myResponseCallback(user);
                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });

}

My EDITED DbService:

public class DbService implements IDbService {

//instance of DbContext for firebase handling
public DbContext dbContext;

public DbService(Context context){
    super();
    dbContext = new DbContext(context);
}

@Override
public User SaveUserProfile(User u) {

    dbContext.AddUserAccount(u);

    return u;
}

@Override
public User GetUserFromUid(String uid) {
    final User[] user1 = {new User()};
    dbContext.GetUserFromFirebase(uid, new Callback() {
        @Override
        public void myResponseCallback(User user) {
            user1[0] = user;
        }
    });
    return user1[0];
}

My EDITED Activity:

    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_details);

    inputName = findViewById(R.id.nameTextView);
    inputEmail = findViewById(R.id.emailTextView);

    FirebaseDatabase mFirebaseInstance = FirebaseDatabase.getInstance();

    //reference to 'users' node
    mFirebaseDatabase = mFirebaseInstance.getReference("users");

    FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();

    assert user != null;
    userId = user.getUid();


    getUserFromFirebase(userId);

}

public void getUserFromFirebase(String uid){
    userDetails = (new DbService(this)).GetUserFromUid(uid);
    inputEmail.setText(userDetails.email);
    inputName.setText(userDetails.name);
}

Debug BEFORE edits:

Debug AFTER edits:

As you can see from the debug, before the callback interface, userDetails was null. After the interface implementation, userDetails is not null but all of the object values are null. I don't know why this is as they are filled in the database. Any ideas?