I am getting quite a lot of these errors in my Tomcat log:

19.12.2012 22:50:36 org.zkoss.zk.ui.impl.UiEngineImpl handleError:1352
SEVERE: >>java.lang.NullPointerException
>> at org.zkoss.zk.ui.event.VisibilityChangeEvent.getVisibilityChangeEvent(VisibilityChangeEvent.java:61)
>> at org.zkoss.zk.ui.impl.DesktopImpl.service(DesktopImpl.java:713)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1194)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:583)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:481)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:489)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
>> at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
>> at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
>> at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
>> at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
>> at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
>> at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
>> at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
>> at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
>> at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:849)
>> at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
>> at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:454)
>> at java.lang.Thread.run(Thread.java:619)

Seems like random pages and random users. One confirmed error was get by an user using Opera, screenshot attached