sum of column in grid not updating the header column

I have read through several posts here but am still have a problem trying to put the sum of a column from multiple rows of a grid into a header column. I have seen code snippets, however, I think I am not arranging my code correctly.

I am still very new to Acumatica coding.    

Below the error message is the full CS code I am using.  Everything validates and compiles, but when I try to access the Cases screen I receive this error message.  So I obviously do not know where and what I need to do here.

Can someone point me in the correct direction?

What I am trying to do is on the Cases screen, use the Relations tab to associate Invoices to a Case and then in a new grid populate that grid with all of the invoice information from the Relations tab. (the other problem I have is that the new grid called “CAM Relations” only lists one invoice at a time, but that is a different problem not for this topic; unless dear reader you know what I am doing wrong there also).

Then I have a UDF in the header which I am trying to populate a grand total of all of the CuryLineTotal column entries in the header column.  Screen shot of the header below as well.

Thank you!
 

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using PX.Common;
using PX.Data;
using System.Collections;
using PX.Data.EP;
using PX.Objects.AR;
using PX.Objects.CT;
using PX.Objects.CR.Workflows;
using PX.Objects.GL;
using PX.Objects.EP;
using PX.Objects.IN;
using PX.Objects.PM;
using PX.SM;
using PX.TM;
using PX.Objects;
using PX.Objects.CR;
using System.Net.Http;
using System.Collections.Generic;
using System.Collections;

namespace PX.Objects.CR
{

  public class CRCaseMaint_Extension : PXGraphExtension<CRCaseMaint>
  {     
          
    #region Event Handlers
        protected virtual void CRCase_RowSelected(PXCache cache, PXRowSelectedEventArgs e, PXRowSelected InvokeBaseHandler)
          {
            InvokeBaseHandler?.Invoke(cache, e);
            var row = (CRCase)e.Row;
            if (row != null)
              {
                if (row.CaseClassID == "CAM")
                  {
                    PXUIFieldAttribute.SetVisible<Objects.CR.CRCaseExt.usrRMAReason>(cache, row, false);
                    PXUIFieldAttribute.SetVisible<Objects.CR.CRCaseExt.usrSupportCategory >(cache, row, false);
                    PXUIFieldAttribute.SetVisible<Objects.CR.CRCaseExt.usrContractDesc>(cache, row, false);
                    PXUIFieldAttribute.SetVisible<Objects.CR.CRCaseExt.usrCaseStage>(cache, row, false);
                    PXUIFieldAttribute.SetVisible<Objects.CR.CRCaseExt.usrIGAHEquipmentID>(cache, row, false);                 
                    PXUIFieldAttribute.SetVisible<Objects.CR.CRCaseExt.usrInvoicesTotal>(cache, row, true);

                  }

              }
            
          }
            

        protected virtual void CRCaseMaint_UsrInvoicesTotal_FieldSelecting(PXCache sender,PXFieldSelectingEventArgs e, PXFieldSelecting InvokeBaseHandler)
        {
          InvokeBaseHandler?.Invoke(sender, e);
          CRCase row = (CRCase)e.Row;
          if (row == null) return; 

decimal? UsrInvoicesTotal = this.CAMRelationsView.Select().FirstTableItems.ToList().Select(x => x.CuryLineTotal).Sum();
        e.ReturnValue UsrInvoicesTotal;  
        }

  
    #endregion
}
          
    // Following view declaration populates the new CAMInfo grid with ARInvoice information          
    #region Selects        public PXSelect<ARInvoice, Where<ARInvoice.noteID, Equal<CurrentValue<CRRelation.targetNoteID>>>> CAMRelationsView;
    #endregion

}