Comment 2 for bug 1741913

Same situations observed on MAAS (DEB install):

ii maas-cli 2.7.1-8262-g.9a49af7df-0ubuntu1~18.04.1 all MAAS client and command-line interface
ii maas-common 2.7.1-8262-g.9a49af7df-0ubuntu1~18.04.1 all MAAS server common files
ii maas-dhcp 2.7.1-8262-g.9a49af7df-0ubuntu1~18.04.1 all MAAS DHCP server
ii maas-proxy 2.7.1-8262-g.9a49af7df-0ubuntu1~18.04.1 all MAAS Caching Proxy
ii maas-rack-controller 2.7.1-8262-g.9a49af7df-0ubuntu1~18.04.1 all Rack Controller for MAAS
ii maas-region-api 2.7.1-8262-g.9a49af7df-0ubuntu1~18.04.1 all Region controller A

Trace:
2020-10-11 06:25:54 -: [critical] WSGI application error
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 87, in callWithContext
self.contexts.pop()
File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 885, in callInContext
return func(*args, **kwargs)
File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 522, in run
self.started = True
--- <exception caught here> ---
File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 500, in run
self.write(elem)
File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 455, in write
self.reactor, wsgiWrite, self.started)
File "/usr/lib/python3/dist-packages/twisted/internet/threads.py", line 122, in blockingCallFromThread
result.raiseException()
File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 385, in raiseException
raise self.value.with_traceback(self.tb)
builtins.AttributeError: 'NoneType' object has no attribute 'writeHeaders'

2020-10-11 06:25:54 asyncio: [error] Exception in callback <function AsyncioSelectorReactor.callLater.<locals>.run at 0x7fda5ce7be18>
handle: <Handle AsyncioSelectorReactor.callLater.<locals>.run>
Traceback (most recent call last):
File "uvloop/cbhandles.pyx", line 47, in uvloop.loop.Handle._run
File "/usr/lib/python3/dist-packages/twisted/internet/asyncioreactor.py", line 290, in run
f(*args, **kwargs)
File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 510, in wsgiError
self.request.loseConnection()
File "/usr/lib/python3/dist-packages/twisted/web/http.py", line 1474, in loseConnection
self.channel.loseConnection()
AttributeError: 'NoneType' object has no attribute 'loseConnection'
...