C# WebBrowser NullReferenceException

Question

I have a problem : I want to check if an account exists on this website : freecraftmaps.fr

Here is my code :

    public static Boolean doesAccountExist(string user, string pass)
    {
        WebBrowser wb = new WebBrowser();
        wb.Navigate("https://www.freecraftmaps.fr/");
        wb.ScriptErrorsSuppressed = true;
        while (wb.ReadyState != WebBrowserReadyState.Complete) { Application.DoEvents(); }
        wb.Document.GetElementById("login").SetAttribute("Value", user);
        wb.Document.GetElementById("password").SetAttribute("Value", pass);
        wb.Document.GetElementById("submit").InvokeMember("click");
        object result = null;
        new Thread(() =>
        {
            System.Threading.Thread.Sleep(200);
            if (wb.Url.AbsoluteUri.EndsWith("erreur.php")) result = false;
            else result = true;
        }).Start();
        while (result == null) { Application.DoEvents(); }
        return (Boolean)result;
    }

The problem is : it only works one time, when I call again the function, it throws an NullReferenceException on :

wb.Document.GetElementById("login").SetAttribute("Value", user);

Thanks,