Cannot insert null value error in custom screen

I have created a custom screen which contains two grids in separate tabs. one grid is a read only grid. while other grid allows to CRUD operations. In this grid I use fully custom DAC. when I trying to insert data, even when insert correct values (not null) for each column sometimes it gives “cannot insert values to columns since null”.  

Even I set values after clicking save button it inserts an empty row as above.
DAC:

using System;
using PX.Data;

namespace EMPPlanningToolR4N5
{
  [Serializable]
  [PXCacheName("PTFillerReelWidths")]
  public class PTFillerReelWidths : IBqlTable
  {
    #region Width
    [PXDBInt(IsKey = true)]
    [PXUIField(DisplayName = "Width")]
    [PXDefault(0)]
    public virtual int? Width { get; set; }
    public abstract class width : PX.Data.BQL.BqlInt.Field<width> { }
    #endregion

    #region PaperTypeID
    [PXDBString(2, IsKey = true, IsFixed = true, InputMask = "")]
    [PXUIField(DisplayName = "Paper Type ID")]
    [PXDefault("##")]
    [PXSelector(typeof(PTPaperTypes.paperTypeID),
        typeof(PTPaperTypes.paperTypeID),
        typeof(PTPaperTypes.paperTypeCD),
        SubstituteKey = typeof(PTPaperTypes.paperTypeID),
        DescriptionField = typeof(PTPaperTypes.description)
        )]
    public virtual string PaperTypeID { get; set; }
    public abstract class paperTypeID : PX.Data.BQL.BqlString.Field<paperTypeID> { }
    #endregion

    #region Gsm
    [PXDBInt(IsKey = true)]
    [PXUIField(DisplayName = "GSM")]
    [PXSelector(typeof(PTFillerReelGSMs.gsm),
        typeof(PTFillerReelGSMs.gsm)
        )]
        public virtual int? Gsm { get; set; }
    public abstract class gsm : PX.Data.BQL.BqlInt.Field<gsm> { }
    #endregion

    #region Priority
    [PXDBInt()]
    [PXUIField(DisplayName = "Priority")]
    public virtual int? Priority { get; set; }
    public abstract class priority : PX.Data.BQL.BqlInt.Field<priority> { }
    #endregion

  }
}

Graph : 
 

using System;
using System.Collections;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using PX.Data;
using PX.Data.BQL.Fluent;

namespace EMPPlanningToolR4N5
{
    public class PTFillerReelWidthListMaint : PXGraph<PTFillerReelWidthListMaint>
    {
        public SelectFrom<PTFillerReelWidths>.View FillerReelWidthView;

        public SelectFrom<PTFillerReelWidthSummary>.View FillerReelWidthSummaryView;

        public PXFilter<PTFillerReelWidthsFilter> ReelWidthFilter;

        public PXFilter<PTFillerReelWidthsFilter> ReelWidthSummaryFilter;

        public PXSave<PTFillerReelWidths> Save;
        public PXCancel<PTFillerReelWidths> Cancel;

        public PTFillerReelWidthListMaint()
        {
            FillerReelWidthSummaryView.AllowDelete = false;
            FillerReelWidthSummaryView.AllowInsert = false;
        }
        public IEnumerable fillerReelWidthSummaryView()
        {
            var query = SelectFrom<PTFillerReelWidths>.View.Select(this).RowCast<PTFillerReelWidths>();
            PTFillerReelWidthsFilter filter = ReelWidthSummaryFilter.Current;
            PXTrace.WriteInformation($"Filter : {filter.Gsm}, {filter.PaperTypeID}");

            if (!string.IsNullOrEmpty(filter?.PaperTypeID))
            {
                query = query.Where(x => x.PaperTypeID == filter.PaperTypeID);
            }
            if (filter?.Gsm != null)
            {
                query = query.Where(x => x.Gsm == filter.Gsm);
            }

                query = query.ToList();

            var groupedData = query
                .GroupBy(x => new { x.PaperTypeID, x.Gsm })
                .Select(g => new PTFillerReelWidthSummary
                {
                    PaperTypeID = g.Key.PaperTypeID,
                    Gsm = g.Key.Gsm,
                    WidthCount = g.Count()
                }).ToList();
            return groupedData;
        }

        public IEnumerable fillerReelWidthView()
        {
            var query = SelectFrom<PTFillerReelWidths>.View.Select(this).RowCast<PTFillerReelWidths>();
            PTFillerReelWidthsFilter filter = ReelWidthFilter.Current;
            PXTrace.WriteInformation($"Filter : {filter.Gsm}, {filter.PaperTypeID}");

            if (!string.IsNullOrEmpty(filter?.PaperTypeID))
            {
                query = query.Where(x => x.PaperTypeID == filter.PaperTypeID);
            }
            if (filter?.Gsm != null)
            {
                query = query.Where(x => x.Gsm == filter.Gsm);
            }

            return query.ToList();
        }
    }


    [PXHidden]
    public class PTFillerReelWidthsFilter : IBqlTable
    {
        #region PaperTypeID
        [PXString]
        [PXUIField(DisplayName = "Paper Type ID")]
        [PXSelector(typeof(PTPaperTypes.paperTypeID),
                typeof(PTPaperTypes.paperTypeID),
                typeof(PTPaperTypes.paperTypeCD),
                SubstituteKey = typeof(PTPaperTypes.paperTypeID),
                DescriptionField = typeof(PTPaperTypes.description)
                )]
        public virtual string PaperTypeID { get; set; }
        public abstract class paperTypeID : PX.Data.BQL.BqlString.Field<paperTypeID> { }
        #endregion

        #region Gsm
        [PXInt]
        [PXUIField(DisplayName = "Gsm")]
        [PXSelector(typeof(PTFillerReelGSMs.gsm),
            typeof(PTFillerReelGSMs.gsm)
            )]
        public virtual int? Gsm { get; set; }
        public abstract class gsm : PX.Data.BQL.BqlInt.Field<gsm> { }
        #endregion
    }
}