list<oppSector__c> bSectorList = new list<oppSector__c>();

  bSectorList = [SELECT Id, Opp__c, SecNum__c,
                    TYPEOF What
                        WHEN Opportunity THEN Id
                        END
                FROM oppSector__c WHERE Opp__c = :stackFlightIDSet];

        system.debug('***************Created List*******' + bSectorList.size());