.Net Framework Web Api SelfHost service stopped suddenly

I created a simple Web Api selfHost as windows service which listens to an address which is dynamically loads from the database and normally it includes port number( like : http://localhost:1900) When I change the address( for example port number, something like http://localhost:1901) the service can catch the requests on the new port but the requests on old port ( http:localhost:1900) leads to crashing the service and it will be stopped. I just could debug my service and saw just NullReference Error and not any more info about it. I don't know even where this error happened and non of my logs could help me. what do you think about this error? Have you ever seen this kind of error before?

For more info I should say just some errors I can see in Event Viewer window :

Application: {Service.exe} Framework Version: v4.0.30319 Description: The process was terminated due to an unhandled exception. Exception Info: System.NullReferenceException at System.Web.Http.SelfHost.HttpSelfHostServer+d__35.MoveNext() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task) at System.Web.Http.SelfHost.HttpSelfHostServer+d__34.MoveNext() at System.Runtime.CompilerServices.AsyncMethodBuilderCore+<>c.b__6_1(System.Object) at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(System.Object) at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean) at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)

Faulting application name: {Service.exe}, version: 1.0.0.0, time stamp: 0xc594704b Faulting module name: KERNELBASE.dll, version: 10.0.14393.3383, time stamp: 0x5ddcb9ff Exception code: 0xe0434352 Fault offset: 0x000dc232 Faulting process id: 0x7370 Faulting application start time: 0x01d72886545b1d41 Faulting application path: {Service PhysicalAddress} Faulting module path: C:\Windows\System32\KERNELBASE.dll Report Id: 305c75f4-8c83-484a-b673-565abfc2b7d6 Faulting package full name: Faulting package-relative application ID

For more details I bring my Service Class Body below :

class service
 {
  HttpSelfHostConfiguration config;
  HttpSelfHostServer server;
  Timer _timer = new Timer();

  protected override void OnStart(string[] args)
  {
      _timer.Interval = 2000;
      _timer.Elapsed += _timer_Elapsed;
      _timer.Enabled = true;
  }

  private void _timer_Elapsed(object sender, ElapsedEventArgs e)
  {
      var listenToUrl = _getDestUrlFromDB();
      var configChanged = false;
      if (config != null && config.BaseAddress.AbsoluteUri != listenToUrl + "/")
      {
          configChanged = true;
          config.Dispose();
      }
      config = new HttpSelfHostConfiguration(uploadApiUrl.Data);
      config.Routes.MapHttpRoute("default",
                                  "api/{controller}/{id}",
                                  new { controller = "Home", id = RouteParameter.Optional });
      config.ClientCredentialType = System.ServiceModel.HttpClientCredentialType.Windows;
      if (server == null)
      {
          server = new HttpSelfHostServer(config);
          var task = server.OpenAsync();
          task.Wait();

      }
      else if (configChanged)
      {

          try
          {
              Process.Start("cmd", $@"netsh http add urlacl url={listenToUrl} ");
              Process.Start("cmd", $@"delete urlacl url={listenToUrl} ");
              server.Dispose();
              server = new HttpSelfHostServer(config);
              var task = server.OpenAsync();
              task.Wait();

          }
          catch (Exception ex)
          {
          }

      }

  }
}