This code for empty collection raised exception at 2nd line:
//"The specified entity is not contained in this EntitySet" exception here
Newly added item created in separate DomainContext (different from QDSCV DomainContext). After QC1.AddNew method call, the new item appeared in QC1 collection and in QC1.SourceCollection. But it does not exists in corresponding EntitySet behind the QC1 DomainContext.