Steps to Reproduce

run the attached file js-redirect.zul

Actual Result

redirect to second page happens.
quite often this leads to an NPE at server side

Nov 27, 2017 12:32:28 PM org.eclipse.jetty.server.HttpChannel handleException
WARNING: /zksupport85/zkwm
java.lang.NullPointerException
	at org.zkoss.zkmax.au.websocket.WSHandshakeRequest.<init>(WSHandshakeRequest.java:55)
	at org.zkoss.zkmax.au.websocket.WebSocketFilter.doFilter(WebSocketFilter.java:41)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1629)

In the network request you can see a status 500 for the /zkwm request

Expected Result

no NPE at serverside (or logged properly if unavoidable)
websocket closed from client side (prevent creation if possible after a redirect)

Debug Info

In the Chrome Network logs the request to /zkwm happens after the rmDeskop command
At serverside the desktop is already gone at that moment causing the NPE.

The error also happens when refreshing a zul multiple times rapidly (requires the right timing)

Workaround

not necessary
The issue doesn't impact a running application since it only happens with already destroyed desktops.