>>java.lang.NullPointerException
>>	at org.zkoss.poi.hssf.usermodel.HSSFCell.setCellStyle(HSSFCell.java:908)
>>	at org.zkoss.poi.hssf.usermodel.HSSFCell.setCellStyle(HSSFCell.java:904)
>>	at org.zkoss.zss.model.sys.impl.BookHelper.setCellStyle(BookHelper.java:3309)
>>	at org.zkoss.zss.model.sys.impl.XRangeImpl.setStyle(XRangeImpl.java:1609)
>>	at org.zkoss.zss.api.impl.RangeImpl.setCellStyle(RangeImpl.java:239)
>>	at org.zkoss.zss.api.impl.RangeImpl.clearStyles(RangeImpl.java:234)
>>	at org.zkoss.zss.ui.impl.undo.ReserveUtil$ReservedResult.restore(ReserveUtil.java:251)
>>	at org.zkoss.zss.ui.impl.undo.AbstractCellDataStyleAction.undoAction(AbstractCellDataStyleAction.java:121)
>>	at org.zkoss.zssex.ui.impl.UndoableActionManagerImpl.undoAction(UndoableActionManagerImpl.java:94)
>>	at org.zkoss.zss.ui.impl.DefaultUserActionManagerCtrl.doUndo(DefaultUserActionManagerCtrl.java:502)
>>	at org.zkoss.zss.ui.impl.DefaultUserActionManagerCtrl$2.processAction(DefaultUserActionManagerCtrl.java:239)
>>	at org.zkoss.zss.ui.impl.ua.AbstractHandler.process(AbstractHandler.java:58)
>>	at org.zkoss.zss.ui.impl.DefaultUserActionManagerCtrl.dispatchKeyAction(DefaultUserActionManagerCtrl.java:324)
>>	at org.zkoss.zss.ui.impl.DefaultUserActionManagerCtrl.onEvent(DefaultUserActionManagerCtrl.java:409)
>>	at org.zkoss.zssex.ui.impl.UserActionManagerCtrlImpl.onEvent(UserActionManagerCtrlImpl.java:285)
>>	at org.zkoss.zss.ui.Spreadsheet$3.onEvent(Spreadsheet.java:385)
>>	at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2748)
>>	at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2719)
>>	at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2660)
>>	at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
>>	at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1720)
>>	at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1505)
>>	at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1215)
>>	at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:600)
>>	at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:482)
>>	at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:491)
>>	at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
>>	at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
>>	at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
>>	at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:401)
>>	at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
>>	at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
>>	at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:766)
>>	at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:450)
>>	at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
>>	at org.mortbay.jetty.Server.handle(Server.java:326)
>>	at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
>>	at org.mortbay.jetty.HttpConnection$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)