NullReferenceException when calling DbContext.Entry

Question

I am facing this strange behavior that sometimes, when i try to Reload a Entry with:

DbEntityEntry<BatchServer> ent = d.Entry(jobServer);
//the ex is thrown at d.Entry
ent.Reload();

EF throws a NullReferenceException Exception. But neither jobServer nor d ( Variable of my Context ) is null and if we take a look into the StackTrace:

System.NullReferenceException: Object reference not set to an instance of an object.
at System.Data.Objects.EntityEntry.DetectChangesInProperty(Int32 ordinal, Boolean detectOnlyComplexProperties, Boolean detectOnly)
at System.Data.Objects.EntityEntry.DetectChangesInProperties(Boolean detectOnlyComplexProperties)
at System.Data.Objects.ObjectStateManager.DetectChangesInScalarAndComplexProperties(IList`1 entries)
at System.Data.Objects.ObjectStateManager.DetectChanges()
at System.Data.Objects.ObjectContext.DetectChanges()
at System.Data.Entity.Internal.InternalContext.DetectChanges(Boolean force)
at System.Data.Entity.Internal.InternalContext.GetStateEntry(Object entity)
at System.Data.Entity.Internal.InternalEntityEntry..ctor(InternalContext internalContext, Object entity)
at System.Data.Entity.DbContext.Entry[TEntity](TEntity entity)
at IEADPC.BatchRemoting.Monitor.ViewModel.MonitorViewModel.<>c__DisplayClass40.<>c__DisplayClass42.<RefreshAll>b__3d(BatchRemotingContexA first chance exception of type 'System.NullReferenceException' occurred in IEADPC.BatchRemoting.Monitor.dll

we see that there is a exception inside of EF. This bug is to 100% reproducible but only after a lot work in the program. So is that a known bug if EF or what am i doing wrong?

Greetings

Update:

Just befor i call this i get all server from the Context:

    DataAccessObject.ClientDataAccess.WorkOnDatabase(d =>
    {
        var colzwei = new ObservableCollection<BatchServer>(d.BatchServers.ToList());
        foreach (var jobServer in colzwei)
        {
            DbEntityEntry<BatchServer> ent;                                 
            ent = d.Entry(jobServer);
            ent.Reload();
            ...
        }
    }

---

    public void WorkOnDatabase(Action<BatchRemotingContext> databaseAction)
    {
        if (DbContext == null)
        {
            using (DbContext = ReturnDatabase())
            {
                databaseAction.Invoke(DbContext);
            }
        }
        else
        {
            databaseAction.Invoke(DbContext);
        }
    }

---

That works well in the whole program, just when i work in my program than it sometimes happens

Answer 1

OK the reason is strange as the bug self.

There is a other entity called: ServerBatch when i added them the first time, everything is OK. Then i edit them and add them the 2nd time to my list of Entitys. After that SaveChanges crash because i try to add the Entity with a PK to a list where the PK already exists. This crash is handled so the program will keep running and right after that crash the Entry method will fail and produce that Exception.