C# Get this error when using background worker

Code

string account = listBox1.Items[i].ToString();

                BackgroundWorker backgroundWorker;

                backgroundWorker = new BackgroundWorker();

                backgroundWorker.DoWork += new DoWorkEventHandler(CheckSpotify);

                backgroundWorker.RunWorkerAsync(account);

                Thread.Sleep(2000);





 private void CheckSpotify(object sender, DoWorkEventArgs e) {



           



            string account = e.Argument.ToString();



            string[] Account = account.Split(':');

            string user = Account[0].ToString();

            string pass = Account[1].ToString();

            WebBrowser w1 = new WebBrowser();



            



            w1.Navigate("https://accounts.spotify.com/en-GB/login?continue=https:%2F%2Fwww.spotify.com%2Fuk%2Faccount%2Foverview%2F");

            HtmlDocument doc = this.w1.Document;

            HtmlElement username = doc.GetElementById("login-username");

            HtmlElement password = doc.GetElementById("login-password");

            username.SetAttribute("value", user );

            password.SetAttribute("value", pass);

            username.Focus();

            SendKeys.SendWait("~");



        }

PHP Code



<?php 
Thread _thread = new Thread(()=> Foo());
_thread.SetApartmentState(ApartmentState.STA);
_thread.Start();

void Foo()
{ 
     // Insert code.
}