System.NullReferenceException: Object reference not set to an instance of an object.

[ppy-sentryintegration]

Don't like the look of this. cc @smoogipoo

Sentry Issue: OSU-SERVER-SPECTATOR-K7

System.NullReferenceException: Object reference not set to an instance of an object.
  File "Hubs/Multiplayer/MultiplayerHub.cs", line 107, in async Task<MultiplayerRoom> MultiplayerHub.joinOrCreateRoom(long roomId, string password, bool isNewRoom)
  File "Hubs/Multiplayer/MultiplayerHub.cs", line 81, in async Task<MultiplayerRoom> MultiplayerHub.CreateRoom(MultiplayerRoom room)
  File "ConcurrentConnectionLimiter.cs", line 106, in async ValueTask<object> ConcurrentConnectionLimiter.InvokeMethodAsync(HubInvocationContext invocationContext, Func<HubInvocationContext, ValueTask<object>> next)
  File "LoggingHubFilter.cs", line 30, in async ValueTask<object> LoggingHubFilter.InvokeMethodAsync(HubInvocationContext invocationContext, Func<HubInvocationContext, ValueTask<object>> next)
...
(2 additional frame(s) were not displayed)

[user:20994839] Failed to invoke hub method: CreateRoom(RoomID:0 Host: Users:0 State:Open Settings: [Name:ggwp1123's awesome room Password:yes Type:HeadToHead Item:0 Queue:AllPlayersRoundRobin Start:00:00:00 AutoSkip:True])

[smoogipoo]

Looks like the sequence of events that happened (in quick succession):

1. User connected
2. User invokes `CreateRoom()` - room starts being created
3. User disconnects - user state is cleaned up
4. `CreateRoom()` finishes creating the room (via shared interop), attempts to join user to room
5. Nullref on accessing user state

Not sure how to fix this at the moment - I wouldn't have expected OnDisconnectedAsync() to be called during execution of other hub methods.

[bdach]

User appears to have had a very unstable connection so maybe #193 would do something here. Otherwise not sure. Probably fine to let it crash this hard because we'd probably have to rethrow our own error anyway.