doorlock-client: Add more error output

Signed-off-by: Ralf Ramsauer <ralf@ramses-pyramidenbau.de>

doorlockd/client/doorlock-client.cpp

@@ -56,6 +56,8 @@ static void onDoorlockUpdate(const Clientmessage &msg)
       LogLevel::info);
     if (mainWindow) {
         mainWindow->setClientmessage(msg);
+    } else {
+        l(LogLevel::error, "No valid UI object!");
     }
 }