I am trying to update a custom field on the SOLine (UsrPOPromisedDate) when the POLine promised date is changed. Below is my graph extension, however SOLine is always null.
When I convert the BQL to T-SQL, I get the expected results returned. Why is my view always returning a null value?

```
using PX.Data;
using PX.KWW.MyProject.DAC;
using PX.Objects.PO;
using PX.Objects.SO;

namespace MyProject.Graph
{
    public class POOrderEntryExt : PXGraphExtension<POOrderEntry>
    {
        public PXSelectJoin<
            SOLine,
            InnerJoin<SOLineSplit,
                On<SOLineSplit.orderType, Equal<SOLine.orderType>,
                And<SOLineSplit.orderNbr, Equal<SOLine.orderNbr>,
                And<SOLineSplit.lineNbr, Equal<SOLine.lineNbr>>>>>,
            Where<SOLineSplit.pOType, Equal<Current<POLine.orderType>>,
                And<SOLineSplit.pONbr, Equal<Current<POLine.orderNbr>>,
                And<SOLineSplit.pOLineNbr, Equal<Current<POLine.lineNbr>>>>>>
            SalesOrderLine;

        protected virtual void _(Events.FieldUpdated<POLine, POLine.promisedDate> eventHandler, PXFieldUpdated baseHandler)
        {
            baseHandler?.Invoke(eventHandler.Cache, eventHandler.Args);

            POLine pOLine = eventHandler.Row;
            if (pOLine is null) return;

            SOLine sOLine = SalesOrderLine.Current;
            SOLineExtension sOLineExtension = sOLine.GetExtension<SOLineExtension>();

            if (sOLine is null || sOLineExtension is null) return;

            sOLineExtension.UsrPOPromisedDate = pOLine.PromisedDate;
            SalesOrderLine.Update(sOLine);
        }
    }
}

```