A "NullPointerException" is thrown with VisibilityChangeEvent which is added as an event listener to the window. The behaviour happened when a browser is having multiple tabs and then after coming back to the application, the error is thrown.

The problem occurs only with the browsers, Microsoft Edge and Opera.

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)