Object referance not set to an instance of an object

private void GridDataSave()
   {
       foreach (GridViewRow s in grdqutotation.Rows)
       {
           ////TextBox id = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_id");

           //TextBox srno = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_srno");

           //TextBox description = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_desc");

           //TextBox Quantity = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_Qty");

           //TextBox Units = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_Unit");

           //TextBox Rate = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_Rate");

           //TextBox Amount = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_amount");
           Tbl_SubFinalQuotation a = new Tbl_SubFinalQuotation();
           ////  TextBox id = (TextBox)s.FindControl("txt_id");
           a.Quto_No = txtQno.Text;
           ////a.id = Convert.ToInt32(s.Cells[0].Text);
           TextBox srno = (TextBox)s.FindControl("txt_srno");
           //Decimal p = Convert.ToDecimal((srno.Text));
           TextBox description = (TextBox)s.FindControl("txt_desc");
          // string q = description.Text;
           TextBox Quantity = (TextBox)s.FindControl("txt_Qty");
           //Decimal r = Convert.ToDecimal((Quantity.Text));
           TextBox Units = (TextBox)s.FindControl("txt_Unit");
          // string t = Units.Text;
           TextBox Rate = (TextBox)s.FindControl("txt_Rate");
          // Decimal u = Convert.ToDecimal((Rate.Text));
           TextBox Amount = (TextBox)s.FindControl("txt_amount");
         //  Decimal v = Convert.ToDecimal((Amount.Text));

           a.srno = Convert.ToDecimal (srno.Text );
           a.productName = description .Text ;
           a.Quantity = Convert.ToDecimal (Quantity .Text );
           a.Units = Units .Text ;
           a.Rate = Convert.ToDecimal (Rate .Text );
           a.Amount = Convert.ToDecimal (Amount .Text );

           context.Tbl_SubFinalQuotations.InsertOnSubmit(a);
           context.SubmitChanges();
           a = null;
       }
   }

private void GridDataSave()
    {
        foreach (GridViewRow s in grdqutotation.Rows)
        {
            ////TextBox id = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_id");

            //TextBox srno = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_srno");

            //TextBox description = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_desc");

            //TextBox Quantity = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_Qty");

            //TextBox Units = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_Unit");

            //TextBox Rate = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_Rate");

            //TextBox Amount = (TextBox)grdqutotation.Rows[s.RowIndex].FindControl("txt_amount");
            Tbl_SubFinalQuotation a = new Tbl_SubFinalQuotation();
            ////  TextBox id = (TextBox)s.FindControl("txt_id");                   
            a.Quto_No = txtQno.Text;
            ////a.id = Convert.ToInt32(s.Cells[0].Text);
            TextBox srno = (TextBox)s.FindControl("txt_srno");
            //Decimal p = Convert.ToDecimal((srno.Text));
            TextBox description = (TextBox)s.FindControl("txt_desc");
           // string q = description.Text;
            TextBox Quantity = (TextBox)s.FindControl("txt_Qty");
            //Decimal r = Convert.ToDecimal((Quantity.Text));
            TextBox Units = (TextBox)s.FindControl("txt_Unit");
           // string t = Units.Text;
            TextBox Rate = (TextBox)s.FindControl("txt_Rate");
           // Decimal u = Convert.ToDecimal((Rate.Text));
            TextBox Amount = (TextBox)s.FindControl("txt_amount");
          //  Decimal v = Convert.ToDecimal((Amount.Text));

            a.srno = Convert.ToDecimal (srno.Text );
            a.productName = description .Text ;
            a.Quantity = Convert.ToDecimal (Quantity .Text );
            a.Units = Units .Text ;
            a.Rate = Convert.ToDecimal (Rate .Text );
            a.Amount = Convert.ToDecimal (Amount .Text );

            context.Tbl_SubFinalQuotations.InsertOnSubmit(a);
            context.SubmitChanges();
            a = null;
        }
    }