Internal server error on accepting cross-posted post previously handled.

When a message is cross-posted and held on more than one list, handling the held post on one list will delete the held message from the message store. The held post is still in the held message view on the other list(s) and viewing, rejecting or discarding the post all behave reasonably, but accepting the post results in

ERROR 2021-06-11 14:48:08,967 middleware 15680 139660351334208 Un-handled exception: HTTP Error 500: {"title": "500 Internal Server Error"}
Traceback (most recent call last):
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/django/views/decorators/http.py", line 40, in inner
    return func(request, *args, **kwargs)
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/postorius/auth/decorators.py", line 63, in wrapper
    return fn(*args, **kwargs)
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/postorius/views/list.py", line 729, in moderate_held_message
    mailing_list.accept_message(msg.request_id)
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/mailmanclient/restobjects/mailinglist.py", line 364, in accept_message
    return self.moderate_message(request_id, 'accept')
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/mailmanclient/restobjects/mailinglist.py", line 334, in moderate_message
    path, data, 'POST')
  File "/opt/mailman/mm/venv/lib/python3.6/site-packages/mailmanclient/restbase/connection.py", line 167, in call
    error_msg, response, None)
urllib.error.HTTPError: HTTP Error 500: {"title": "500 Internal Server Error"}