Can't view or modify list of rooms in a community

[logic]

Description

I have a community defined on my homeserver, and viewing it in riot-web gives a spinner forever on the list of rooms, and am unable to add rooms to the community.

Steps to reproduce

In riot, go to "view community", see spinner under the "ROOMS" heading. On the server, synapse throws:

synapse[4356]: synapse.http.server: [GET-136] Failed handle request via 'GroupRoomServlet': <XForwardedForRequest at 0x7fac2448e810 method='GET' uri=
'/_matrix/client/r0/groups/%2Bclub140%3Alogic.net/rooms' clientproto='HTTP/1.0' site=8008>
                                                        Traceback (most recent call last):
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
                                                            result = g.send(result)
                                                        StopIteration: {}
                                                        
                                                        During handling of the above exception, another exception occurred:
                                                        
                                                        Traceback (most recent call last):
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
                                                            result = g.send(result)
                                                        StopIteration: {}
                                                        
                                                        During handling of the above exception, another exception occurred:
                                                        
                                                        Traceback (most recent call last):
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/synapse/http/server.py", line 77, in wrapped_request_handler
                                                            await h(self, request)
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/synapse/http/server.py", line 326, in _async_render
                                                            callback_return = await callback_return
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
                                                            result = result.throwExceptionIntoGenerator(g)
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/twisted/python/failure.py", line 512, in throwExceptionIntoGenerat
or
                                                            return g.throw(self.type, self.value, self.tb)
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/synapse/rest/client/v2_alpha/groups.py", line 347, in on_GET
                                                            group_id, requester_user_id
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
                                                            result = result.throwExceptionIntoGenerator(g)
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/twisted/python/failure.py", line 512, in throwExceptionIntoGenerat
or
                                                            return g.throw(self.type, self.value, self.tb)
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/synapse/groups/groups_server.py", line 519, in get_rooms_in_group
                                                            room_id, len(joined_users), with_alias=False, allow_private=True
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
                                                            result = g.send(result)
                                                          File "/srv/matrix.logic.net/venv3.7/lib64/python3.7/site-packages/synapse/handlers/room_list.py", line 294, in generate_room_entry
                                                            result["m.federate"] = create_event.content.get("m.federate", True)
                                                        AttributeError: 'NoneType' object has no attribute 'content'

Version information

• Homeserver: matrix.logic.net

• Version: 1.5.1

• Install method: pip (virtualenv)

• Platform: Fedora 31 x86_64 on a DigitalOcean VM

[logic]

Aaaaaaaand, a little more searching shows that this is just the same thing as #5824, and the solution there seems to have fixed the problem for me.