流程报错，未将对象引用设置

System.NullReferenceException: 未将对象引用设置到对象的实例。
在 Kingdee.BOS.Serialization.DcxmlSerializerReadImplement.ReadElement(IDataEntityType dt, Object entity)
在 Kingdee.BOS.Serialization.DcxmlSerializerReadImplement.DoCollectionPropertyAction(String action, ICollectionProperty property, Object entity)
在 Kingdee.BOS.Serialization.DcxmlSerializerReadImplement.ReadCollectionProperty(ICollectionProperty property, Object entity)
在 Kingdee.BOS.Serialization.DcxmlSerializerReadImplement.ReadElement(IDataEntityType dt, Object entity)
在 Kingdee.BOS.Serialization.DcxmlSerializerReadImplement.DoComplexPropertyAction(String action, IComplexProperty property, Object entity)
在 Kingdee.BOS.Serialization.DcxmlSerializerReadImplement.ReadElement(IDataEntityType dt, Object entity)
在 Kingdee.BOS.Serialization.DcxmlSerializer.ReadElement(XmlReader reader, IDataEntityType dt, Object entity)
在 Kingdee.BOS.Serialization.DcxmlSerializer.DeserializeFromString(String xml, Object entity)
在 Kingdee.BOS.App.Core.MetaDataLoadService.Add(AbstractBusinessMetadata baseMetadata, MetaDataNode node)
在 Kingdee.BOS.App.Core.MetaDataLoadService.GetMetaData(AbstractBusinessMetadata baseMetadata, MetaDataNode node, String[] subNodeIds)
在 Kingdee.BOS.App.Core.MetaDataLoadService.GetMetaData(AbstractBusinessMetadata baseMetadata, MetaDataNode node, String[] subNodeIds)
在 Kingdee.BOS.App.Core.MetaDataLoadService.GetMetaData(AbstractBusinessMetadata baseMetadata, MetaDataNode node, String[] subNodeIds)
在 Kingdee.BOS.App.Core.MetaDataLoadService.Load(String layoutViewId)
在 Kingdee.BOS.App.Core.MetadataXmlReader.LoadFromCache(String id, Boolean bGetBaseObject, Int32 modelTypeId)
在 Kingdee.BOS.App.Core.MetadataXmlReader.Load(String id, Boolean runtime, Boolean loadEmbedRefObject, Boolean getBaseObject, Int32 modeltypeId, Boolean isLoadSubObject)
在 Kingdee.BOS.App.Core.MetaServicePlugIn.FormMetaServicePlugIn.CreateFieldRefProperties(FormMetadata formMetaData)
在 Kingdee.BOS.App.Core.MetaServicePlugIn.FormMetaServicePlugIn.OnLoadEmbedRefObject(AfterReadArgs e)
在 Kingdee.BOS.App.Core.PlugInProxy.MetadtaServicePlugInProxy.FireOnLoadEmbedRefObject()
在 Kingdee.BOS.App.Core.MetaDataLoadService.LoadEmbedRefObject(AbstractBusinessMetadata meta)
在 Kingdee.BOS.App.Core.MetadataXmlReader.LoadEmbedRefObject(String id, String layoutViewId, Boolean runtime, Boolean getBaseObject, Int32 modeltypeId, Boolean isLoadSubObject, AbstractBusinessMetadata meta, MetaDataLoadService loadService)
在 Kingdee.BOS.App.Core.MetadataXmlReader.Load(String id, Boolean runtime, Boolean loadEmbedRefObject, Boolean getBaseObject, Int32 modeltypeId, Boolean isLoadSubObject)
在 Kingdee.BOS.Workflow.App.Core.Repositories.BillRepository.GetBill(Context ctx, Object pkValue, String formId)
在 Kingdee.BOS.Workflow.Engine.ProcessContext.get_BOSBill()
在 Kingdee.BOS.Workflow.WorkflowDynamicContext.TryGetBillFieldValue(String key, Object& value, Int32 row)
在 Kingdee.BOS.Workflow.WorkflowDynamicContext.GetField(String key)
在 Kingdee.BOS.Workflow.WorkflowDynamicContext.TryGetWFField(String key, Object& value)
在 Kingdee.BOS.Scripting.DynamicText.Parse(ExpressionContext context)
在 Kingdee.BOS.Workflow.ActionEvents.ApprovalAction.GetTitle(ActionEventContext ctx, WorkflowDynamicContext ec)
在 Kingdee.BOS.Workflow.ActionEvents.ApprovalAction.CreateReceiver(ActionEventContext ctx, IList`1 userList, DynamicObjectCollection consignRuleList)
在 Kingdee.BOS.Workflow.ActionEvents.ApprovalAction.BuildAssign(ActionEventContext ctx, IList`1 userList, DynamicObjectCollection consignRuleList)
在 Kingdee.BOS.Workflow.ActionEvents.ApprovalAction.Execute(ActionEventContext ctx)
在 Kingdee.BOS.Workflow.Elements.ActionEvent.ExecuteActionEvent(ActionEventContext ctx)
在 Kingdee.BOS.Workflow.Kernel.Runtime.MapStateExecutor.ExecuteActionEvents()
在 Kingdee.BOS.Workflow.Kernel.Runtime.MapStateExecutor.ExecuteMapState()
在 Kingdee.BOS.Workflow.Kernel.Runtime.MapSchedular.ExecuteMapStateInstance(IMapStateInstance instance)
在 Kingdee.BOS.Workflow.Kernel.Runtime.MapSchedular.Run()

求大神支招，流程使用一直很正常，突然就报错了，做流程测试也是正常通过，问友问题，谢谢！