/// <summary>
/// Grades the submitted questionnaire and saves the evaluation to database, throws NullReferenceException if the student hasn't already submitted a questionnaire.
/// </summary>
public void Grade()
{
Evaluation = new Evaluation(DateTime.Now, Questionnaire.Evaluate(), Questionnaire.Topic.ID, Submitter.ID, QuestionnaireSession.ID);
Evaluation.SaveAsync();
}