I struggled for a few hours with this error.
Eventually, I understood what I was doing wrong.

... or -at least- not just me!

I was following the Channels documentation trying to implement some Subscription behaviour using Django.
I actually read the documentation wrong so I was trying to open a new WebSocket connection by simply instantiate it in the browser as follow:

const socket = new WebSocket("ws://localhost:8000/graphql/");

...and promptly every time I ran this function, the server logged this:

INFO:     ('172.20.0.1', 45834) - "WebSocket /graphql/" 403
INFO:     connection rejected (403 Forbidden)
INFO:     connection closed
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/uvicorn/protocols/websockets/websockets_impl.py", line 247, in run_asgi
    result = await self.app(self.scope, self.asgi_receive, self.asgi_send)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/channels/routing.py", line 62, in __call__
    return await application(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/channels/routing.py", line 116, in __call__
    return await application(
           ^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/channels/consumer.py", line 94, in app
    return await consumer(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/channels/consumer.py", line 58, in __call__
    await await_many_dispatch(
  File "/usr/lib/python3.11/site-packages/channels/utils.py", line 50, in await_many_dispatch
    await dispatch(result)
  File "/usr/lib/python3.11/site-packages/strawberry/channels/handlers/base.py", line 88, in dispatch
    await super().dispatch(message)
  File "/usr/lib/python3.11/site-packages/channels/consumer.py", line 73, in dispatch
    await handler(message)
  File "/usr/lib/python3.11/site-packages/channels/generic/websocket.py", line 238, in websocket_disconnect
    await self.disconnect(message["code"])
  File "/usr/lib/python3.11/site-packages/strawberry/channels/handlers/ws_handler.py", line 116, in disconnect
    await self._handler.handle_disconnect(code)
          ^^^^^^^^^^^^^
AttributeError: 'GraphQLWSConsumer' object has no attribute '_handler'

subscription SubscribeToChatRooms {
  joinChatRooms(
    rooms: [{ roomName: "room1" }, { roomName: "room2" }]
    user: "foo"
  ) {
    roomName
    message
    currentUser
  }
}

Now... I was wrong... Definitely!

BUT... Is that really the proper error message to log in this case?
It -rather- seems like it crashes badly due to some bug while trying to deal with this bad behavior.

Probably, it should check if self._handler is actually instantiated before using it or -better- it shouldn't call disconnect before actually connecting.

In the future, I would prefer not to receive an unclear (and apparently serious) Sentry notification every time a bot tries to open a WebSocket connection with my server... 😉