Not thread-safe.

def _handle_chat_event(self, event: events.ChatMessageWasReceived) -> None:
        """
        Not thread-safe.

        """
        for subscriber in self._chat_subscribers:
            try:
                subscriber(event)
            except Exception:
                LOG.exception(self._prefix_log_message(
                    f"failed to send chat event {event} to "
                    f"subscriber {subscriber}"
                ))