NullPointerException after postEvent()

public void onClick$btnCreateTask(Event evt) {
   ...
   Events.postEvent(new Event(Events.ON_CHANGE)); 
}

14.12.2011 09:34:49 org.zkoss.zk.ui.impl.UiEngineImpl handleError:1256
SCHWERWIEGEND: >>java.lang.NullPointerException
>>	at org.zkoss.spring.webflow.mvc.servlet.ZkFlowControllerListener.afterProcessEvent(ZkFlowControllerListener.java:99)
>>	at org.zkoss.zk.ui.impl.EventInterceptors.afterProcessEvent(EventInterceptors.java:160)
>>	at org.zkoss.zk.ui.util.Configuration.afterProcessEvent(Configuration.java:2469)
>>	at org.zkoss.zk.ui.impl.DesktopImpl.afterProcessEvent(DesktopImpl.java:1177)
>>	at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:139)
>>	at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1612)
>>	at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1396)
>>	at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1109)
>>	at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:569)
>>	at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:463)
>>	at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:471)
>>	at javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
>>	at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
>>	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
>>	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
>>	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
>>	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
>>	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
>>	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
>>	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
>>	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:851)
>>	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
>>	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
>>	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:278)
>>	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
>>	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
>>	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
>>	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
>>	at java.lang.Thread.run(Unknown Source)