Read Text File From Internet

hi, i use this code for read text file from internet, but When the user clicks on the button The program is closed.

using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using System.Net;
using System.IO;

 Button button = FindViewById<Button>(Resource.Id.button1);
            var txt = FindViewById<TextView>(Resource.Id.textView1);

            button.Click += (sender, e) =>
            {
                txt.Text = ShowRemoteStringFile("http://shadoow.ir/adress/BNJfilmha.txt");
                AlertDialog.Builder builder = new AlertDialog.Builder(this);
                builder.SetTitle("AriaVPN Address Finder");
                builder.SetIcon(Android.Resource.Drawable.IcDialogAlert);
                builder.SetMessage("New Address Found : \n " + txt.Text);
                builder.Show();
            };

private string ShowRemoteStringFile(string url)
        {
            WebRequest req = WebRequest.Create(url);
            WebResponse rsp = (HttpWebResponse)req.GetResponse();
            StreamReader rdr = new StreamReader(rsp.GetResponseStream());
            string result = rdr.ReadToEnd();
            return result;
        }