在从不同列表进行查询时，未将对象引用设置为对象的实例

我有下面的linq。由于我从两个数据上下文中进行查询，所以我将这些表分解为var列表。但是，我出现了错误"Object reference not set to a instance of a Object"。发生这种情况是因为edr为null。

var meetingsQuery = (from s in this.ModelContext.Meetings select s).ToList(); 
var deliverablesQuery = (from s in this.ModelContext.Deliverables select s).ToList(); 
var deliverableDatesQuery = (from s in this.ModelContext.DeliverableDates select s).ToList();
var refDateTypesQuery = (from s in this.ModelContext.RefDateTypes select s).ToList(); 
var refDeliverablesQuery = (from s in this.ModelContext.RefDeliverables select s).ToList();
var updatesQuery = (from s in this.ArenaUpdateBASEModelContext.Updates select s).ToList(); 
var updateQCsQuery = (from s in this.ArenaUpdateBASEModelContext.UpdateQCs select s).ToList();
var submissionUpdates = (from e in meetingsQuery 
                         from edr in deliverablesQuery.Where(dr => dr.MeetingId == e.MeetingId && !dr.DeletedFlag).DefaultIfEmpty()
                                 from ed in deliverableDatesQuery.Where(d => d.DeliverableId == edr.DeliverableId && !d.DeletedFlag && d.RefDateTypeId == 1).DefaultIfEmpty()
                                 from ed2 in deliverableDatesQuery.Where(d2 => d2.DeliverableId == edr.DeliverableId && !d2.DeletedFlag && d2.RefDateTypeId == 2).DefaultIfEmpty()
                                 join ret in refDateTypesQuery on ed.RefDateTypeId equals ret.RefDateTypeId
                                 join rdt in refDeliverablesQuery on edr.RefDeliverableId equals rdt.RefDeliverableId
                                 join upd in updatesQuery on edr.RefDsgnSubmissionTypeId equals upd.UpdateId
                                 join uqc in updateQCsQuery on upd.UpdateId equals uqc.UpdateId
                                 where
                                        !e.DeletedFlag && !ret.DeletedFlag && !rdt.DeletedFlag && !upd.DeletedFlag && !uqc.DeletedFlag && e.ProjectId == arenaPiD// && rdt. .ObjectIdLink == "Update_UpdateId"
                                        && uqc.RefQCId == 6  // Distributed
                                        && uqc.RefQCStatusId == 2 // Complete
                                 orderby e.ScheduledDT descending
                                 select new
                                 {
                                     e.MeetingId,
                                     e.ScheduledDT,
                                     edr.DeliverableId,
                                     edr.RefDeliverableId,
                                     rdt.DeliverableAbbrv,
                                     UpdateId = edr.RefDsgnSubmissionTypeId != null ? edr.RefDsgnSubmissionTypeId : 0,
                                     RefRecommendationId = upd.RefRecommendationId != null ? upd.RefRecommendationId : 0,
                                     uqc.RefQCId,
                                     uqc.RefQCStatusId,
                                     DeadlineDate = ed != null ? ed.DeliverableDateValue.ToString() : "",
                                     ActualDate = ed2 != null ? ed2.DeliverableDateValue.ToString() : ""
                                 }).ToList();