Dynamics 365 + Custom Workflow + Entity Reference cannot have Id and Key Attributes empty

I have 2 custom workflows. The output of one becomes the input for the another.

After I call the first custom workflow and use the output of it in another step, it gives me error:

'InValid Argument' error - Entity Reference cannot have Id and Key Attributes empty.

Code for 1st workflow -

public class RetrieveCaseForUnit : WorkFlowActivityBase
{
    #region Input Parameters
    [Input("Unit")]
    [ReferenceTarget(msdyn_customerasset.EntityLogicalName)]
    public InArgument<EntityReference> Unit { get; set; }
    #endregion

    #region Output Parameters
    [Output("Case")]
    [ReferenceTarget(Incident.EntityLogicalName)]
    public OutArgument<EntityReference> Case{ get; set; }
    #endregion


    public override void ExecuteCRMWorkFlowActivity(CodeActivityContext executionContext, LocalWorkflowContext crmWorkflowContext)
    {
        try
        {

            if (crmWorkflowContext == null)
            {
                throw new ArgumentNullException("crmWorkflowContext is null");
            }

            crmWorkflowContext.Trace("Getting Unit Input");
            EntityReference unitRef = Unit.Get<EntityReference>(executionContext);
            if (unitRef == null)
            {
                crmWorkflowContext.Trace("Error Message : Unit value not provided");
                throw new ArgumentNullException("Unit value not provided");
            }

            EntityReference caseRef = GetCase(crmWorkflowContext, unitRef);

            if (caseRef != null)
            {
                Case.Set(executionContext, caseRef);
            }
            else
            {
                Case.Set(executionContext, null);
            }
        }
        catch (Exception ex)
        {
            throw new InvalidWorkflowException();
        }
    }

    private static EntityReference GetCase(LocalWorkflowContext crmWorkflowContext, EntityReference unitRef)
    {
        EntityReference caseRef = null;

        CrmServiceContext serviceContext = new CrmServiceContext(crmWorkflowContext.OrganizationService);
        var caseRecord = (from currentcase in serviceContext.IncidentSet
                          where currentcase.gsscore_Unitid.Id == unitRef.Id 
                              &&  currentcase.gsscore_CaseTypeId.Id == new Guid("3A152B94-D2DF-E711-A94A-000D3A30DB97")
                          orderby currentcase.CreatedOn descending
                          select currentcase).FirstOrDefault();

        crmWorkflowContext.Trace("Case Record" + caseRecord);

        if (caseRecord != null)
            caseRef = caseRecord.ToEntityReference();

        return caseRef;
    }
}

Code for 2nd workflow -

public class NMSGetWorkOrderForCase : WorkFlowActivityBase
{
    #region Input Parameters
    [Input(gsscore_nmsmessage.Fields.gsscore_CaseId)]
    [ReferenceTarget(Incident.EntityLogicalName)]
    public InArgument<EntityReference> Case { get; set; }
    #endregion

    #region Output Parameters
    [Output(gsscore_nmsmessage.Fields.WorkOrder)]
    [ReferenceTarget(msdyn_workorder.EntityLogicalName)]
    public OutArgument<EntityReference> NMSWorkOrder { get; set; }
    #endregion

    public override void ExecuteCRMWorkFlowActivity(CodeActivityContext executionContext, LocalWorkflowContext crmWorkflowContext)
    {
        if (crmWorkflowContext == null)
        {
            throw new ArgumentNullException("crmWorkflowContext is Null");
        }


        if (Case.Get(executionContext) == null)
        {
            crmWorkflowContext.UserId = crmWorkflowContext.WorkflowExecutionContext.UserId;
            throw new InvalidWorkflowException();
        }

        crmWorkflowContext.Trace("Start");
        Guid caseId = Case.Get<EntityReference>(executionContext).Id;
        try
        {
            CrmServiceContext serviceContext = new CrmServiceContext(crmWorkflowContext.OrganizationService);
            var WorkOrderRecord = (from currentworkoder in serviceContext.msdyn_workorderSet
                                   where currentworkoder.msdyn_ServiceRequest.Id == caseId
                                   orderby currentworkoder.CreatedOn descending
                                   select currentworkoder.Id
                                      ).ToList().FirstOrDefault();

           if (WorkOrderRecord != null)
            {
                EntityReference workorderRef = new EntityReference(msdyn_workorder.EntityLogicalName, WorkOrderRecord);
                NMSWorkOrder.Set(executionContext, workorderRef);
            }
        }
        catch (Exception ex)
        {
            crmWorkflowContext.TracingService.Trace("Case record does not exist." + crmWorkflowContext.WorkflowExecutionContext.MessageName + ex.Message);
            if (crmWorkflowContext.ErrorCode == null)
            {
                crmWorkflowContext.ErrorCode = ((int)WorkflowActivityErrorCode.NMSGetWorkOrderForCaseError).ToString();
            }
            crmWorkflowContext.UserId = crmWorkflowContext.WorkflowExecutionContext.UserId;
            throw new InvalidWorkflowException();
        }

    }
}

WorkOrderRecord record is a GUID. It can't be null. if (WorkOrderRecord != null) should be changed to if (WorkOrderRecord != Guid.Empty)