//Working fine here!
Questionnaire questionnaireeeee = context.Set<Questionnaire>().Include(q => q.QuestionnaireQuestions.Single(q => q.ID == 1);
if(expiredQuestionnaire != null)
{
    RedirectToAction("Administrator", "CalculateResults", new { Questionnaire = expiredQuestionnaire });
}
//need to see who is logged in, administrator or professor
 string userId = User.Identity.GetUserId();
//...
if(User.IsInRole("Administrator"))
{
    //grouping subject instances by same semester
    var subjectInstancesBySemester = from subjectInstances in context.SubjectInstances group subjectInstances by subjectInstances.Semester;
    foreach (var subjectInstancesGroup in subjectInstancesBySemester)
    {
         // getting questionnaire related to this semester
         if(subjectInstancesGroup.Key.QuestionnaireID != null)
         {
        //Exception is here!
             Questionnaire questionnaire = context.Set<Questionnaire>().Include(q => q.QuestionnaireQuestions).Single(q => q.ID == (int) subjectInstancesGroup.Key.QuestionnaireID);
             //...
         }
    }
}