My popup keeps getting called (loop)

@joe21 

I would recommend not removing the PXPersistingCheck property value.

Have you tried using a direct PXException instead?

       protected virtual void CRCase_RowPersisting(PXCache sender, PXRowPersistingEventArgs e)
        {
            if (e.Row == null)
            {
                return;
            }
            CRCase caseRow = (CRCase)e.Row;

            CRCaseSSGPortalExt cRCaseExt = PXCache<CRCase>.GetExtension<CRCaseSSGPortalExt>(caseRow);

            if (cRCaseExt.UsrSerialNbr == null)
            {
                throw new PXException(ActionsMessages.Warning);
			}
        }

@Fernando Amadoz 

I did that and it works.

//sender.RaiseExceptionHandling<CRCaseSSGPortalExt.usrSerialNbr>(caseRow, null, new PXSetPropertyException(ISOMessages.PopupSerialNumber, PXErrorLevel.Error));

However, I want the popup to show to really clarify to the user that they can enter TBD if they don’t know the serial number.  

If I could find a way to use e.Cancel without having the popup loop, that would be ideal.

@joe21 I had a similar issue but for a different use case. As you know, If the keyboard's delete key is hit and a field is not in edit mode, Acumatica deletes the whole row in grids. In Project Cost Projection  I wanted user confirmation and if No was selected undo the change. I guess you can slightly edit the code for the Persisting Event and Custom Field Ext validation and dialog Message Buttons. Hope this helps.

protected virtual void PMCostProjectionLine_RowDeleting(PXCache cache, PXRowDeletingEventArgs e, PXRowDeleting BaseHandler)
{
    if (e.Row == null) return;

    PMCostProjectionLine row = e.Row as PMCostProjectionLine;

    if (Base.Details.Ask(HCLPMMessages.Confirmation, HCLPMMessages.DeleteCostProjectionLine, MessageButtons.YesNo) != WebDialogResult.Yes)
    {
        e.Cancel = true;
        return;
    }

    BaseHandler?.Invoke(cache, e);
}

@joe21 I did a quick revision here. Hopefully, it works or helps.

// Verify when persisting:
protected virtual void CRCase_RowPersisting(PXCache cache, PXRowPersistingEventArgs e, PXRowPersisting BaseHandler)
{
    if (e.Row == null) return;

    CRCase caseRow = e.Row as CRCase;
    CRCaseSSGPortalExt caseExt = PXCache<CRCase>.GetExtension<CRCaseSSGPortalExt>(caseRow);

    if (caseExt.UsrSerialNbr == null)
    {
        Base.Case.Ask("Message", MessageButtons.OK);
        e.Cancel = true;
        return;
    }

    BaseHandler?.Invoke(cache, e);
}

// or you can write a persist delegate like (I have not tested this but should work):
public delegate void PersistDelegate();
[PXOverride]
public void Persist(PersistDelegate del)
{
    if (Base.Case.Current != null)
    {
        object serialNbr = Base.Case.Current.GetExtension<CRCaseSSGPortalExt>().UsrSerialNbr;
        if (serialNbr == null)
        {
            Base.Case.Cache.RaiseFieldVerifying<CRCaseSSGPortalExt.usrSerialNbr>(Base.Case.Current, ref serialNbr);
            Base.Case.Ask("Message", MessageButtons.OK);
            return;
        }
    }

    del();
}