Make up for forgotten parentheses

lib/tfw/event_handlers/event_handler_base.py

@@ -86,7 +86,7 @@ class EventHandlerBase(ABC):
         :param message: the message received
         :returns: the message to send back
         """
-        raise NotImplementedError
+        raise NotImplementedError()
 
     def send_message(self, message):
         self.server_connector.send_message(message, self.scope)