MVC application SESSION for some users null - asp.net-mvc

Question

My problem is with a system i developed for school. We are experiencing some issues for some users. this is the error:
[NullReferenceException: Object reference not set to an instance of an object.]
_360Feedback.Controllers.SurveyController.Answers() +170
lambda_method(Closure , ControllerBase , Object[] ) +78
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +273
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +38
System.Web.Mvc.Async.<>c__DisplayClass39.<BeginInvokeActionMethodWithFilters>b__33() +119
System.Web.Mvc.Async.<>c__DisplayClass4f.<InvokeActionMethodFilterAsynchronously>b__49() +452
System.Web.Mvc.Async.<>c__DisplayClass37.<BeginInvokeActionMethodWithFilters>b__36(IAsyncResult asyncResult) +15
System.Web.Mvc.Async.<>c__DisplayClass2a.<BeginInvokeAction>b__20() +33
System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult asyncResult) +240
System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__18(IAsyncResult asyncResult) +28
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +15
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +53
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +15
System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__3(IAsyncResult asyncResult) +42
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +15
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +606
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +288
For some reason the student id is not retained by the session object. I cannot regenerate the error so it is very hard for me to find the reason what is causing this.
Setting the session to cookieless seemed to cause less errors for users, but still generated the error for some users.
I can see that for about 90% of the users everything is working fine. I will post the controller code where this session id is used. I hope you guys can help me.
public ActionResult Start(string x)
{
string decryptedstudentIDString = EncryptionHelper.Decrypt(x);
int studentID = Convert.ToInt32(decryptedstudentIDString);
Student student = repo.FindStudentWithID(studentID);
Session["StudentName"] = student.Name;
Session["StudentID"] = student.StudentID;
if (student.CompletedAnswers == true)
{
return RedirectToAction("Completed");
}
return View(student);
}
public ActionResult Answers()
{
Student student = repo.FindStudentWithID(Convert.ToInt32(Session["StudentID"]));
if (student.CompletedAnswers == true)
{
return RedirectToAction("Completed");
}
List<Student> remainingStudentInGroup = repo.GetRemainingStudentIDsInGroup(student.StudentID);
student.Answers.Add(new AnswerSet() { AssociatedStudentID = student.StudentID, AssociatedStudentName = student.Name });
foreach (var studentInGroup in remainingStudentInGroup)
{
student.Answers.Add(new AnswerSet() { AssociatedStudentID = studentInGroup.StudentID, AssociatedStudentName = repo.FindStudentWithID(studentInGroup.StudentID).Name });
}
Group group = repo.GetGroupAssociatedWithStudent(student.StudentID);
Session["StudentID"] = student.StudentID;
Session["GroupSize"] = group.Students.Count;
return View(student);
}
[HttpPost]
public ActionResult Answers(Student student)
{
int s = repo.FindStudentWithID(Convert.ToInt32(Session["StudentID"])).StudentID;
foreach (var answerSet in student.Answers)
{
repo.AddAnswerSetToStudent(answerSet, s);
}
return RedirectToAction("Completed");
}
public ActionResult Completed()
{
Student student = repo.FindStudentWithID(Convert.ToInt32(Session["StudentID"]));
if (student.CompletedAnswers == false)
{
repo.SetBooleanForCompletedAnswerToTrue(student.StudentID);
Group group = repo.GetGroupAssociatedWithStudent(student.StudentID);
ExcelHelper.FillExcelWithAnswersForStudent(group.ExcelFilePath, student);
ExcelHelper.CheckIfAllStudentAnswered(group);
}
return View();
}
Error occurs when users submit form, StudentID is null when they post there answers.