exception occurred during run

here is my code asp.net web service

   [WebMethod]
        public void host()
        {
            var siteName = "Default Web Site";
            var appPath = "mywebapp";

            var serverManager = new ServerManager();
            var site = serverManager.Sites[siteName];
            appPath = appPath.StartsWith("/") ? appPath : "/" + appPath;            var app = site.Applications[appPath];
            var urls = new List<string>();

            foreach (var binding in site.Bindings)
            {
                var sb = new StringBuilder();
                sb.Append(binding.Protocol);
                sb.Append("://");
                if (!string.IsNullOrWhiteSpace(binding.Host))
                {
                    sb.Append(binding.Host);
                }
                else
                {
                    if (Equals(binding.EndPoint.Address, IPAddress.Any))
                    {
                        sb.Append("localhost");
                    }
                    else
                    {
                        sb.Append(binding.EndPoint.Address);
                    }
                }

                if (binding.EndPoint.Port != 80)
                {
                    sb.Append(":");
                    sb.Append(binding.EndPoint.Port);
                }

                sb.Append(app.Path);
                urls.Add(sb.ToString());
            }

            
            HttpContext.Current.Response.ContentType = "application/json";
            HttpContext.Current.Response.Write(JsonConvert.SerializeObject(urls));
            HttpContext.Current.Response.End();
        }

i get this exception at high light code 

System.NullReferenceException: 'Object reference not set to an instance of an object.'