netty: random java.nio.channels.ClosedChannelException: null while closing websocket

Random error while closing a web socket:

```
WARN  [2017-03-19 19:02:47,428] [nio-worker-3-2] An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception.
java.nio.channels.ClosedChannelException: null
	at io.netty.channel.AbstractChannel$AbstractUnsafe.write(...)(Unknown Source)
```