null reference exemption

Question

am getting the errror of " object reference not set to an instance of an object" at this line of code:

 

SqlConnection MyCon = new SqlConnection(ConfigurationManager.ConnectionStrings[Excel107ConString].ConnectionString);

 

below is the whole code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Data;
using System.Data.OleDb;
using System.Data.SqlClient;
using System.Configuration;
        public partial class WebForm1 : System.Web.UI.Page
        {
            protected void Upload(object sender, EventArgs e)
            {

                string excelPath = Server.MapPath("~/Files/") + Path.GetFileName(FileUpload1.PostedFile.FileName);
                FileUpload1.SaveAs(excelPath);

                string conString = string.Empty;
                string extension = Path.GetExtension(FileUpload1.PostedFile.FileName);
                switch (extension)
                {

                    case ".xls":
                        SqlConnection MyCon = new SqlConnection(ConfigurationManager.ConnectionStrings[Excel107ConString].ConnectionString);
                       break;

                }
                conString = string.Format(conString, excelPath);
                using (OleDbConnection excel_con = new OleDbConnection(conString))
                {

                    excel_con.Open();
                    string sheet1 = excel_con.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null).Rows[0]["TABLE_NAME"].ToString();
                    DataTable dtExcelData = new DataTable();


                    dtExcelData.Columns.AddRange(new DataColumn[3] { new DataColumn("InvoiceNumber", typeof(string)),
                new DataColumn("AmountPaid", typeof(decimal)),
                new DataColumn("Remarks",typeof(string)) });

                    using (OleDbDataAdapter oda = new OleDbDataAdapter("SELECT * FROM [" + sheet1 + "]", excel_con))
                    {
                        oda.Fill(dtExcelData);
                    }
                    excel_con.Close();

                    string consString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
                    using (SqlConnection con = new SqlConnection(consString))
                    {
                        using (SqlBulkCopy sqlBulkCopy = new SqlBulkCopy(con))
                        {

                            sqlBulkCopy.DestinationTableName = "smarttable";

                           sqlBulkCopy.ColumnMappings.Add("InvoiceNumber", "InvoiceNumber");
                            sqlBulkCopy.ColumnMappings.Add("AmountPaid", "AmountPaid");
                            sqlBulkCopy.ColumnMappings.Add("Remarks", "Remarks");
                            con.Open();
                            sqlBulkCopy.WriteToServer(dtExcelData);
                            con.Close();
                        }
                    }
                }
            }

}