public class MainWindowViewModel : ViewModelBase
{
    ViewModelBase _content;
    private Database _db;

    public MainWindowViewModel(Database db)
    {
        _db = db;
        CreateNewUser();
    }

    public ViewModelBase Content
    {
        get => _content;
        private set => this.RaiseAndSetIfChanged(ref _content, value);
    }

    public void CreateNewUser()
    {
        var vm = new SelectUserViewModel();
        var vm2 = new HomeScreenViewModel();

        Observable.Merge(
            vm.Login,
            vm.CreateUser
            .Select(_ => (User)null))
            .Take(1)
            .Subscribe(model =>
            {
                if (model != null)
                {
                    UserQuery.InsertNewUsername(model);
                    Content = vm2;
                }
                else
                {
                    Content = vm;
                }
            });

        Content = vm;
    }
}