Windows 7 Object reference not set to an instance of an object." Getting this error message

private void btn_Send_Click(object sender1, EventArgs e1)
        {
            String result;
            string apiKey = "Zs3TRsRIFCs-tRyNcGbbKpVO8otloaz2lspUS44tBk";
            string numbers = txt_Contactno.Text;
            string message = txt_MessageBox.Text;
            string sender = "DASHAN";

            String url = "https://api.textlocal.in/send/?apikey=" + apiKey + "&numbers=" + numbers + "&message=" + message + "&sender=" + sender;
            //refer to parameters to complete correct url string

            StreamWriter myWriter = null;         
            HttpWebRequest objRequest = (HttpWebRequest)WebRequest.Create(url);

            objRequest.Method = "POST";
            objRequest.ContentLength = Encoding.UTF8.GetByteCount(url);
            objRequest.ContentType = "application/x-www-form-urlencoded";
            try
            {
                myWriter = new StreamWriter(objRequest.GetRequestStream());
                myWriter.Write(url);
            }
            catch (Exception e)
            {

            }
            finally
            {
                myWriter.Close();
            }
            HttpWebResponse objResponse = (HttpWebResponse)objRequest.GetResponse();
            using (StreamReader sr = new StreamReader(objResponse.GetResponseStream()))
            {
                result = sr.ReadToEnd();
                // Close and clean up the StreamReader
                sr.Close();
                MessageBox.Show("Message Sent");
            }

        }