diff --git a/BeWo/ServiceProxy/ServiceFacade.cs b/BeWo/ServiceProxy/ServiceFacade.cs
index d4a506770..334fe9bea 100644
--- a/BeWo/ServiceProxy/ServiceFacade.cs
+++ b/BeWo/ServiceProxy/ServiceFacade.cs
@@ -709,14 +709,14 @@ namespace BeWo.ServiceProxy
 			DispatcherPriority.Normal,
 			(Action)delegate
 			{
-				pCallback(t);
+				pCallback?.Invoke(t);
 			});
 
 			Action<Exception> tErrorCallback = (t) => dispatcher.BeginInvoke(
 			DispatcherPriority.Normal,
 			(Action)delegate
 			{
-				pErrorCallback(t);
+				pErrorCallback?.Invoke(t);
 			});
 
 			DoAsync<TClient, TInterface, T>(pFunc, tCallback, tErrorCallback, showWaitDialog);