Showing a custom field in Process Shipments that could come from different DACs

I’m trying to do this exact thing with a different custom field.  However, I get “Sequence contains no elements” error on this line:

SOOrderShipment.FK.Shipment.SelectChildren(Base, shipment)?.First();

I also need to do this on the SOShipmentEntry graph and tried this:

public class SOShipmentEntryExt : PXGraphExtension<SOShipmentEntry>
{
    public static bool IsActive() => true;

    protected virtual void _(Events.RowSelected<SOShipment> e)
    {
        SOShipment row = e.Row;

        if (row != null)
        {
            SOOrderShipment.FK.Shipment.SelectChildren(Base, row)?.First();

            var soOrderExt = row.GetExtension<SOOrderExt>();

            var shipmentExt = row.GetExtension<SOShipmentExt>();

            shipmentExt.UsrNeedPurchaseOrder = soOrderExt?.UsrNeedPurchaseOrder;

        }

    }

}

But I get the same error on the SelectChildren line.

I was also originally trying to use PXDBScalar but could not get to work reliably everywhere or would sometimes show true on the wrong SOShipment:

#region UsrNeedPurchaseOrder
[PXDBScalar(typeof(Search2<SOOrderExt.usrNeedPurchaseOrder,
       LeftJoin<SOOrderShipment, On<SOShipment.shipmentNbr, Equal<SOOrderShipment.shipmentNbr>>>,
    Where<SOOrderShipment.shipmentNbr, Equal<Current<SOShipment.shipmentNbr>>>,
       OrderBy<Desc<SOOrderExt.usrNeedPurchaseOrder>>>))]
PXBool]
PXUIField(DisplayName = "Need PO", IsReadOnly = true)]
public virtual bool? UsrNeedPurchaseOrder { get; set; }
public abstract class usrNeedPurchaseOrder : PX.Data.BQL.BqlBool.Field<usrNeedPurchaseOrder> { }
#endregion

So I was really happy when I found this solution, but unfortunately I can’t figure out why it’s not working/crashing either.

UPDATE: changed to use FirstOrDefault() - not crashing now but still not quite right. I’ll keep trying

UPDATE 2: Success!

public class SOInvoiceShipmentExt : PXGraphExtension<SOInvoiceShipment>
{
    public static bool IsActive() => true;

    public IEnumerable orders()
    {
        foreach (SOShipment shipment in Base.orders())
        {
            var soShipment = SOOrderShipment.FK.Shipment.SelectChildren(Base, shipment)?.FirstOrDefault();

            if (soShipment != null)
            {
                SOOrder objOrder = PXSelectJoin<SOOrder, InnerJoin<SOOrderShipment, On<SOOrder.orderType, Equal<SOOrderShipment.orderType>,
                And<SOOrder.orderNbr, Equal<SOOrderShipment.orderNbr>>>>,
                Where<SOOrderShipment.shipmentNbr, Equal<Required<SOOrderShipment.shipmentNbr>>>>.
                Select(Base, soShipment.ShipmentNbr);

                var soOrderExt = objOrder.GetExtension<SOOrderExt>();

                var shipmentExt = shipment.GetExtension<SOShipmentExt>();

                shipmentExt.UsrNeedPurchaseOrder = soOrderExt?.UsrNeedPurchaseOrder;

                yield return shipment;

            }

        }
    }
}