c# HttpResponseMessage throws exception HttpRequestException: ... ---> WebException: The remote name could not be resolved: 'www.thexlr.com' at ...

I tires to build crawler that separate websites with problems as (redirected, not fond, website is down ...) from list. I added reachtextbox to store url list and another rtb to display html code for url-s. I need to rewrite some informations that belong to problem url-s and skip it in list to crawl other url-s. Some websites throws Exception as belove and stops crawelling process:

System.Net.Http.HttpRequestException: An error accurred while sending the request. ---> System.Net.WebException: The remote name could not be resolved: 'www.thexlr.com' at HttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at HttpClientHandler.GetResponseCallback(IAsyncResult ar) ...

I do not know how to deal with problem websites. Do someone have any idea?

My code:

        HttpClient clientKontrKat;
        HttpResponseMessage responseKontrKat;

        #region start button

        public async void buttonSpustitKontroluStranok_Click(object sender, EventArgs e)
        {
            richTextBoxKontrolaUrlWebObsah.Text = "";
            richTextBoxKontrolaUrlVnutorneLinky.Text = "";

            StringBuilder sb = new StringBuilder();

            if (dataGridView1.Rows.Count > -1)
            {
                if (categoryComboBox3.Items.Count > -1 && categoryComboBox3.SelectedItem != null && statusComboBox1.Items.Count > -1 && statusComboBox1.SelectedItem != null)
                {
                    string categItemCoPrehliadnut = categoryComboBox3.SelectedValue.ToString();
                    string statusItemCoOznacit = statusComboBox1.SelectedValue.ToString();

                    foreach (DataGridViewRow item in dataGridView1.Rows)
                    {
                        string ColumnCategItemsCoPrehliadnut = item.Cells[2].Value.ToString();

                        if (categItemCoPrehliadnut == ColumnCategItemsCoPrehliadnut)
                        {
                            string urlCoPrehliadnut = item.Cells[0].Value.ToString();

                            sb.Append(urlCoPrehliadnut + Environment.NewLine);
                        }
                    }
                }
            }
            richTextBoxKontrolaUrlVnutorneLinky.Text = sb.ToString();
            richTextBoxKontrolaUrlVnutorneLinky.Text = string.Join(Environment.NewLine, richTextBoxKontrolaUrlVnutorneLinky.Lines.Distinct());

            #region Crawler

            int i = 0;
            while (i < richTextBoxKontrolaUrlVnutorneLinky.Lines.Length - 1)
            {
                try
                {
                    buttonSpustitKontroluStranok.Enabled = false;
                    string lineUrl = richTextBoxKontrolaUrlVnutorneLinky.Lines[i].ToString();

                    HttpClientHandler handler = new HttpClientHandler();
                    handler.AllowAutoRedirect = false;

                    clientKontrKat = new HttpClient(handler);
                    clientKontrKat.MaxResponseContentBufferSize = 256000;
                    clientKontrKat.DefaultRequestHeaders.Add("user-agent", "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)");
                    responseKontrKat = await clientKontrKat.GetAsync(lineUrl);
                    //responseKontrKat.EnsureSuccessStatusCode();
                    string responseBody = await responseKontrKat.Content.ReadAsStringAsync();
                    // Above three lines can be replaced with new helper method in following line 
                    // string body = await client.GetStringAsync(uri);
                    labelKontrolaWebRespPopis.Text = "Web response popis: " + responseKontrKat.StatusCode;

                    if (responseKontrKat.IsSuccessStatusCode)
                    {
                        richTextBoxKontrolaUrlWebObsah.Text = "";
                        richTextBoxKontrolaUrlWebObsah.Text = responseBody;
                    }
                    else
                    {
                        // problems handling here
                        //if (richTextBoxKontrolaUrlVnutorneLinky.Lines.Contains(lineUrl))
                        //{
                            //MessageBox.Show("stop, " + lineUrl + " i: " + i);
                            richTextBoxUrlsToCrawl.SelectionStart = i;
                            richTextBoxUrlsToCrawl.SelectionLength = richTextBoxUrlsToCrawl.Text.IndexOf("\n", 0) + 1;
                            richTextBoxUrlsToCrawl.SelectedText = "";

                        //}
                    }

                    i++;
                    int pocetLiniekNaPrehliadnutie = richTextBoxKontrolaUrlVnutorneLinky.Lines.Count() - 1;
                    labelPocetStranok.Text = "Pocet prehliadnutych stranok:" + i + " z " + pocetLiniekNaPrehliadnutie;

                    //       // MessageBox.Show("stop, " + lineUrl);
                    //    }
                    //}
                }
                catch (WebException ex)
                {
                    //if (ex.Status == WebExceptionStatus.ProtocolError)
                    //{
                    //    if (((HttpWebResponse)ex.Response).StatusCode == HttpStatusCode.NotFound)
                    //    {
                    //        // handle the 404 here
                    //    }
                    //}
                    //else 
                    if (ex.Status == WebExceptionStatus.NameResolutionFailure)
                    {
                        // handle name resolution failure
                        MessageBox.Show("stop, ");
                    }
                }
                catch (Exception ExKontStranok)
                {
                    //if (responseKontrKat.IsSuccessStatusCode)
                    //{
                    //    return;
                    //}
                    //if (responseKontrKat.StatusCode == HttpStatusCode.NotFound)
                    //if (ExKontStranok.GetBaseException().InnerException(WebException )==WebExceptionStatus .NameResolutionFailure)
                    //{
                    MessageBox.Show("stop, ");
                        richTextBoxUrlsToCrawl.SelectionStart = i;
                        richTextBoxUrlsToCrawl.SelectionLength = richTextBoxUrlsToCrawl.Text.IndexOf("\n", 0) + 1;
                        richTextBoxUrlsToCrawl.SelectedText = "";
                    //}
                        MessageBox.Show("chyba:\n" + ExKontStranok.ToString());
                }
                finally
                {
                    buttonSpustitKontroluStranok.Enabled = true;
                }
            }

            #endregion

            MessageBox.Show("Urobene!");

        }

        #endregion

        #region stop button

        private void buttonZastavitKontroluStranok_Click(object sender, EventArgs e)
        {
            clientKontrKat.CancelPendingRequests();
            responseKontrKat.Dispose();
        }

        #endregion

Thank you for help if any.