Avoid a NullPointerException in NetServer.onError

[simonlindholm]

I saw the following stack trace:

Exception in thread "WebsocketSelector14" java.lang.NullPointerException
at battlecode.server.NetServer.onError(NetServer.java:165)
at org.java_websocket.server.WebSocketServer.handleFatal(WebSocketServer.java:439)
at org.java_websocket.server.WebSocketServer.run(WebSocketServer.java:287)
at java.lang.Thread.run(Thread.java:745)

and indeed the first argument to onError can sometimes be null, so this seems like a reasonable fix.

[jbloxham]

How did you obtain this error? I was able to reproduce this exception by starting two instances of the server simultaneously, but this produces fairly seriously erroneous behavior across the server, and is in general not supported.

[simonlindholm]

Yeah, I think I got that from running two servers at the same time (trying to parallelize testing).