public SelecionarProfessorViewModel(Avaliacao avaliacao)
        {
            IsVisible   = true;
            Professores = new ObservableCollection <Professor>();
            using (var dao = new ProfessorDAO())
            {
                var profs = dao.GetAtivos();

                if (profs != null)
                {
                    Professores = new ObservableCollection <Professor>(profs);
                }
            }

            SelecionarCommand = new Command(() => {
                IsVisible    = false;
                var selected = Professores?.Where(x => x.Selected).ToList();

                if (selected.Count < 1)
                {
                    App.Current.MainPage?.DisplayAlert("Selecione o professor", "Nenhum professor selecionado.", "Fechar");
                    IsVisible = true;
                    return;
                }
                var pages = new Views.AvaliarProfessorPage(avaliacao, selected);
                App.Current.MainPage?.Navigation.PushAsync(pages);
                IsVisible = true;
            });
        }