RuntimeError: Set changed size during iteration

[julien-duponchelle]

https://sentry.io/gns3/gns3-server/issues/215901459/

CancelledError: 
(1 additional frame(s) were not displayed)
...
  File "aiohttp/streams.py", line 640, in read
    result = yield from super().read()
  File "aiohttp/streams.py", line 475, in read
    yield from self._waiter
  File "asyncio/futures.py", line 379, in __iter__
    yield self  # This tells Task to wait for completion.
  File "asyncio/tasks.py", line 297, in _wakeup
    future.result()
  File "asyncio/futures.py", line 284, in result
    raise CancelledError

RuntimeError: Set changed size during iteration
  File "gns3server/web/route.py", line 196, in control_schema
    yield from func(request, response)
  File "gns3server/handlers/api/controller/project_handler.py", line 232, in notification
    yield from project.close()
  File "gns3server/controller/project.py", line 539, in close
    for compute in self._project_created_on_compute:

RuntimeError: Set changed size during iteration