Add counter for the number of times pick list printed

I have added a custom field in SOPickingWorksheetShipment DACExtension which stores how many times action to print pick list in screen (SO302500) Picking Worksheets. 
Not able to save my values to the DB.

DACExtension

#region UsrPickListPrintCount
    [PXDBInt]
    [PXUIField(DisplayName="Pick List Print Count")]

    public virtual int? UsrPickListPrintCount { get; set; }
    public abstract class usrPickListPrintCount : PX.Data.BQL.BqlInt.Field<usrPickListPrintCount> { }
    #endregion

GraphExtension

public class SOPickingWorksheetReview_Extension : PXGraphExtension<PX.Objects.SO.SOPickingWorksheetReview>
    {
        #region Actions
        [PXButton(IsLockedOnToolbar = true), PXUIField(DisplayName = "Print Pick Lists")]
        public virtual IEnumerable printPickList(PXAdapter a)
        {
            SOPickingWorksheetReview graph = PXGraph.CreateInstance<SOPickingWorksheetReview>();
            SOPickingWorksheet row = Base.worksheet.Current;
            SOPickingWorksheetShipment worksheetShipment = Base.shipmentLinks.Current;
            SOPickingWorksheetShipmentExt worksheetShipmentExt = worksheetShipment.GetExtension<SOPickingWorksheetShipmentExt>();

            if (worksheetShipmentExt.UsrPickListPrintCount >= 2)
            {
                if (Base.worksheet.Ask("Do you want to reprint this pick list?", MessageButtons.OKCancel)
             != WebDialogResult.OK) return a.Get();
            }

            //update print count
            worksheetShipmentExt.UsrPickListPrintCount = worksheetShipmentExt.UsrPickListPrintCount ?? 0;
            worksheetShipmentExt.UsrPickListPrintCount = worksheetShipmentExt.UsrPickListPrintCount + 1;
            graph.shipmentLinks.Cache.SetValueExt<SOPickingWorksheetShipmentExt.usrPickListPrintCount>(worksheetShipment, worksheetShipmentExt.UsrPickListPrintCount);
            graph.shipmentLinks.Update(worksheetShipment);
            Base.Save.Press();

            return Base.printPickList(a);
        }
        public PXAction<SOPickingWorksheet> PrintPickList;
        #endregion
    }