Object reference not set to an instance of an object?

private List<dailyelectricity> InsertDailyElectricity(string machine, string type, string value)
    {

        List<dailyelectricity> electricityDetails = new List<dailyelectricity>();

        SqlConnection con = new SqlConnection(connectionString);
        SqlCommand cmd = new SqlCommand("InsertDailyElectricityDetails", con);
        cmd.CommandType = CommandType.StoredProcedure;
        
        for (int i = 0; i < WebDataGrid1.Rows.Count; i++)
        {
            cmd.Parameters.Add("@machine", machine);
            cmd.Parameters.Add("@readtype", type);
            cmd.Parameters.Add("@value", value);                
        }


        con.Open();
        SqlDataReader read;

        try
        {
            read = cmd.ExecuteReader();

            while (read.Read())
            {
                electricityDetails.Add(new DailyElectricity() { MachineName = read.GetString(0), ReadingType = read.GetString(1), ReadingValue = read.GetString(2)  });
            }

        }
        catch (Exception)
        {
            throw;
        }
        finally
        {
            con.Close();
        }
     return electricityDetails;
    }

InsertDailyElectricity(WebDataGrid1.Rows[0].Items[1].Value.ToString(), WebDataGrid1.Rows[0].Items[1].Value.ToString(), WebDataGrid1.Rows[0].Items[1].Value.ToString());