Floods of `Cannot read property 'close' of null` on timeouts occurring occasionally

```
TypeError: Cannot read property 'close' of null
    at WebSocketReceiver.heartbeat_timeout (/root/transport/node_modules/sockjs/lib/trans-websocket.js:157:26)
    at null._onTimeout (/root/transport/node_modules/sockjs/lib/trans-websocket.js:75:24)
    at Timer.listOnTimeout (timers.js:92:15)
```
