Attribute ID Selector Determines Field Values in Custom Field

I have created a line level field in AP303000 form and I need to have Attribute ID selector 

namespace MidayaCustomizations
{
    public class APTranExt : PXCacheExtension<PX.Objects.AP.APTran>
    {
        #region UsrAttributeSelection
        [PXDBString(250)]
        [PXUIField(DisplayName = "Attribute ID")]
        [PXSelector(typeof(Search<CSAttributeDetail.attributeID,Where<CSAttributeDetail.attributeID, Equal<CSAttribute.attributeID>>>),
         typeof(CSAttributeDetail.attributeID))]  
        public virtual string UsrAttributeSelection { get; set; }
        public abstract class usrAttributeSelection : PX.Data.BQL.BqlString.Field<usrAttributeSelection> { }
        #endregion

        #region UsrAttributeValues
        [PXDBString(250)]
        [PXUIField(DisplayName = "Attribute Values")]

        public virtual string UsrAttributeValues { get; set; }
        public abstract class usrAttributeValues : PX.Data.BQL.BqlString.Field<usrAttributeValues> { }
        #endregion
    }
}

And I need to get the Attribute ID from that field and in the UsrAttributeValues field, the values from that particular attribute should be populated (eg- Attribute ID “CSS” is a dropdown field and in UsrAttributeValues field these list of values should be available to be selected). 

protected void APTran_UsrAttributeSelection_FieldUpdated(PXCache cache, PXFieldUpdatedEventArgs e, PXFieldUpdated InvokeBaseHandler)
        {
            InvokeBaseHandler?.Invoke(cache, e);
            APTran row = (APTran)e.Row;
            if (row == null) return;
            var rowExt = PXCache<APTran>.GetExtension<APTranExt>(row);

            List<string> allowedValues = new List<string>();
            List<string> allowedLabels = new List<string>();

            if (rowExt != null)
            {
                foreach (CSAttributeDetail objCSAttributeDetail in PXSelect<CSAttributeDetail,
                                                          Where<CSAttributeDetail.attributeID, Equal<Required<APTranExt.usrAttributeSelection>>>>.Select(Base, rowExt.UsrAttributeSelection))


                {
                    allowedValues.Add(objCSAttributeDetail.ValueID);
                    allowedLabels.Add(objCSAttributeDetail.Description);
                }
                e.ReturnState = PXStringState.CreateInstance(e.ReturnState, 10, true, typeof(APTranExt.usrAttributeValues).Name, false, -1, string.Empty, allowedValues.ToArray(), allowedLabels.ToArray(), false, null);
            }

        }

Currently trying to use a field updated event, but I am facing errors, any help would be appreciated.