Reproducing Steps:

1. open zss.test/issue3/493-deleteLastSheet.zul
2. click the button to delete all sheets
3. When you delete the last sheet

Expected:
A message dialog shown it's forbidden.

Actual:
A message dialog shown and an exception occurs:

2013/10/28 下午 03:22:03 org.zkoss.zk.ui.impl.UiEngineImpl handleError:1362
嚴重的: >>java.lang.IllegalArgumentException: Sheet index (-1) is out of range (0..-1)
>> at org.zkoss.poi.xssf.usermodel.XSSFWorkbook.validateSheetIndex(XSSFWorkbook.java:1143)
>> at org.zkoss.poi.xssf.usermodel.XSSFWorkbook.getSheetAt(XSSFWorkbook.java:934)
>> at org.zkoss.zss.model.sys.impl.XSSFBookImpl.getWorksheetAt(XSSFBookImpl.java:412)
>> at org.zkoss.zss.model.sys.impl.XRangeImpl.initCellRef(XRangeImpl.java:101)
>> at org.zkoss.zss.model.sys.impl.XRangeImpl.<init>(XRangeImpl.java:91)
>> at org.zkoss.zss.model.sys.XRanges.newRange(XRanges.java:245)
>> at org.zkoss.zss.model.sys.XRanges.range(XRanges.java:200)
>> at org.zkoss.zssex.ui.ValidationHelper.validate(ValidationHelper.java:43)
>> at org.zkoss.zssex.ui.impl.DataValidationHandlerImpl.validate(DataValidationHandlerImpl.java:105)
>> at org.zkoss.zss.ui.Spreadsheet.processStopEditing0(Spreadsheet.java:4527)
>> at org.zkoss.zss.ui.Spreadsheet.processStopEditing(Spreadsheet.java:4504)
>> at org.zkoss.zss.ui.Spreadsheet.access$47(Spreadsheet.java:4502)
>> at org.zkoss.zss.ui.Spreadsheet$2.onEvent(Spreadsheet.java:353)
>> at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2743)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2714)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2655)
>> at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1720)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1505)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1215)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:601)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:483)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:492)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
>> at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
>> at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:401)
>> at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
>> at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
>> at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:766)
>> at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:450)
>> at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
>> at org.mortbay.jetty.Server.handle(Server.java:326)
>> at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
>> at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:945)
>> at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
>> at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
>> at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
>> at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410)
>> at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

Then any characters you type, the same exception is thrown.