update ข้อมูลจาก datagridview ลงดาต้าเบส แต่มันขึ้น error ว่า Object reference not set to an instance of an object. แก้ยังงัยค่ะ

 Private Sub btnnext_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnnext.Click

        check_radio()

        If DataGridView1.RowCount <= 0 Then Exit Sub
        Dim countDVG1 As Integer = 0
        For i As Integer = DataGridView1.RowCount - 1 To 0 Step -1
            If IsDBNull(DataGridView1.Rows(i).Cells("custpay").Value) Or IsDBNull(DataGridView1.Rows(i).Cells("discount").Value) Then
                countDVG1 = countDVG1 + 1
            End If
        Next

        If rdoBB.Checked = False And rdoKB.Checked = False And rdoTMB.Checked = False Then
            MessageBox.Show("     กรุณาเลือก เลขที่บัญชีการโอน     ", "คำเตือน", MessageBoxButtons.OK, MessageBoxIcon.Stop)
        ElseIf txtpay.Text = "" Then
            MessageBox.Show("     กรุณาระบุจำนวนยอดชำระ     ", "คำเตือน", MessageBoxButtons.OK, MessageBoxIcon.Stop)
        ElseIf countDVG1 > 0 Then
            MessageBox.Show("     กรุณากรอกระบุยอดชำระและหมายเหตุ ให้ครบถ้วน     ", "คำเตือน", MessageBoxButtons.OK, MessageBoxIcon.Stop)
        ElseIf lbltotalpay.Text <> txtpay.Text Or lbltotalRemain.Text <> lblramain.Text Then
            MessageBox.Show("  ยอดเงินในช่องชำระไม่ตรงกับยอดเงินโอนรวม     ", "คำเตือน", MessageBoxButtons.OK, MessageBoxIcon.Stop)
        Else

            If MessageBox.Show("     คุณต้องการที่จะบันทึกข้อมูลนี้หรือไม่ ?     ", "บันทึก", MessageBoxButtons.YesNo, MessageBoxIcon.Question) = vbYes Then

                'Dim Gen_billno As String
                Dim billnumberDGV As String
                Dim payDGV As Decimal
                Dim remainDGV As Decimal
                Dim choieDGV As String
                Dim remarkDGV As String
                timetick = DateTimePicker2.Value

                Try
                    conjen = New SqlConnection(strcon)
                    comjen = New SqlCommand("select MAX(billNo) from " & sale_name & "", conjen)

                    dajen = New SqlDataAdapter(comjen)
                    dajen.Fill(dsjen, "data")

                    jen = CInt(dsjen.Tables("data").Rows(0)(0)) + 1
                Catch ex As Exception
                    jen = 11111
                End Try

                billreport = jen
                For j As Integer = DataGridView1.RowCount - 1 To 0 Step -1

                    billnumberDGV = DataGridView1.Rows(j).Cells("billnumber").Value
                    payDGV = Convert.ToDecimal(DataGridView1.Rows(j).Cells("custpay").Value)
                    remainDGV = Convert.ToDecimal(DataGridView1.Rows(j).Cells("discount").Value)

                    If IsDBNull(DataGridView1.Rows(j).Cells("choice_remake").Value) Then
                        choieDGV = ""
                    Else
                        choieDGV = DataGridView1.Rows(j).Cells("choice_remake").Value
                    End If

                    If IsDBNull(DataGridView1.Rows(j).Cells("remark").Value) Then
                        remarkDGV = ""
                    Else
                        remarkDGV = DataGridView1.Rows(j).Cells("remark").Value
                    End If

                    'Dim tr As SqlTransaction = consave.BeginTransaction
                    'Try
                    'comsave = New SqlCommand("UPDATE pu SET checkbox = @checkbox , billno = @billno , cashtotal = @cashtotal , datemake =@datemake , billtotal = @billtotal ,arrears = @arrears,choice_remake = @choice_remake , remark =@remark ,datetrasfer = @datetrasfer , bank = @bank , pay = @pay WHERE billnumber = '" & app.Cells.Item("billnumber").Value & "' ", consave)

                    'Dim tr As SqlTransaction = consave.BeginTransaction

                    comsave = New SqlCommand("UPDATE " & sale_name & " SET checkbox = @checkbox , billno = @billno  , payment = @payment , datemake =@datemake , transfertotal = @transfertotal ,billtotal= @billtotal ,discount = @discount,datetrasfer = @datetrasfer ,bank = @bank , checknumber = @checknumber , pay = @pay WHERE billnumber = '" & billnumberDGV & "' ", consave)

                    comsave.Parameters.AddWithValue("@checkbox", "2")
                    comsave.Parameters.AddWithValue("@billno", jen)
                    comsave.Parameters.AddWithValue("@payment", payDGV.ToString("###,###,##0.00"))
                    comsave.Parameters.AddWithValue("@datemake", Now.ToString())
                    comsave.Parameters.AddWithValue("@transfertotal", lbltotalpay.Text)
                    comsave.Parameters.AddWithValue("@billtotal", sum.ToString("###,###,##0.00"))
                    comsave.Parameters.AddWithValue("@discount", remainDGV.ToString("###,###,##0.00"))
                    comsave.Parameters.AddWithValue("@datetrasfer", timetick)
                    comsave.Parameters.AddWithValue("@checknumber", checknumber)
                    comsave.Parameters.AddWithValue("@bank", bank)
                    comsave.Parameters.AddWithValue("@pay", payby)
                    consave.Open()
                    comsave.ExecuteNonQuery()
                    consave.Close()
                    'tr.Commit()
                    'Catch ex As Exception
                    'tr.Rollback()
                    'MessageBox.Show("     บันทึกข้อมูลไม่ได้     ", "บันทึก", MessageBoxButtons.OK, MessageBoxIcon.Information)
                    'End Try
                Next
                MessageBox.Show("     บันทึกข้อมูลเรียบร้อยเเล้ว     ", "บันทึก", MessageBoxButtons.OK, MessageBoxIcon.Information)
                DataGridView1.Rows.Clear()
                'DataGridView1.Hide()
                frmReport.Show()
            End If
        End If
    End Sub