Fixed startup racing issue

Emby.Dlna/Main/DlnaEntryPoint.cs

@@ -227,9 +227,12 @@ namespace Emby.Dlna.Main
         private void StartDeviceDiscovery(ISsdpCommunicationsServer communicationsServer)
         {
             try
+            {
+                if (communicationsServer != null)
                 {
                     ((DeviceDiscovery)_deviceDiscovery).Start(communicationsServer);
                 }
+            }
             catch (Exception ex)
             {
                 _logger.LogError(ex, "Error starting device discovery");