System.InvalidOperationException: Nullable object must have a value.
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at System.Nullable`1.get_Value()
at UvA.DataNose.Data.GradeInputEntry.<>c__DisplayClass28_0.<get_IsVURelevant>b__0(Enrolment z)
at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
at UvA.DataNose.Data.GradeInputEntry.get_IsVURelevant()
at UvA.DataNose.Data.UvAData.<>c.<AddVisibility>b__32_0(GradeInputEntry e)
at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
at UvA.DataNose.Data.UvAData.AddVisibility(GradeInputList entity, Int32[] years)
at UvA.DataNose.Data.UvAData.OnSavingChanges()
at UvA.DataNose.Data.UvAData.SaveChanges()
at UvA.DataNose.Core.DataContainer.SaveData(Boolean notify, UvAData data, String filePath, Int32 lineNumber, Boolean noTracking)
Maybe the exam date should be set anyway (say to the last date?), and/or we need to check all places where this is used?