SEVERE: >>java.lang.RuntimeException: Zero length string is an invalid sheet name
>>	at org.zkoss.poi.ss.formula.SheetNameFormatter.needsDelimiting(SheetNameFormatter.java:102)
>>	at org.zkoss.poi.ss.formula.SheetNameFormatter.appendFormat(SheetNameFormatter.java:61)
>>	at org.zkoss.poi.ss.formula.ptg.ExternSheetNameResolver.prependSheetName(ExternSheetNameResolver.java:44)
>>	at org.zkoss.poi.ss.formula.ptg.Ref3DPtg.toFormulaString(Ref3DPtg.java:91)
>>	at org.zkoss.poi.ss.formula.FormulaRenderer.toFormulaString(FormulaRenderer.java:92)
>>	at org.zkoss.poi.hssf.model.HSSFFormulaParser.toFormulaString(HSSFFormulaParser.java:83)
>>	at org.zkoss.poi.hssf.usermodel.HSSFCell.getCellFormula(HSSFCell.java:624)
>>	at org.zkoss.zss.model.sys.impl.BookHelper.getEditText(BookHelper.java:1132)
>>	at org.zkoss.zss.model.sys.impl.BookHelper.getRichEditText(BookHelper.java:1197)
>>	at org.zkoss.zss.ui.impl.XUtils.getRichEditText(XUtils.java:360)
>>	at org.zkoss.zss.ui.impl.XUtils.getEditText(XUtils.java:351)
>>	at org.zkoss.zss.ui.impl.XUtils.getEditText(XUtils.java:81)
>>	at org.zkoss.zss.ui.Spreadsheet$ExtraCtrl.getCellAttr(Spreadsheet.java:3251)
>>	at org.zkoss.zss.ui.Spreadsheet$ExtraCtrl.getRangeAttrs(Spreadsheet.java:3086)
>>	at org.zkoss.zss.ui.Spreadsheet.createActiveRange(Spreadsheet.java:1691)
>>	at org.zkoss.zss.ui.Spreadsheet.updateSheetAttributes(Spreadsheet.java:933)
>>	at org.zkoss.zss.ui.Spreadsheet.setSelectedSheetDirectly(Spreadsheet.java:879)
>>	at org.zkoss.zss.ui.Spreadsheet.access$43(Spreadsheet.java:862)
>>	at org.zkoss.zss.ui.Spreadsheet$ExtraCtrl.setSelectedSheetDirectly(Spreadsheet.java:3678)
>>	at org.zkoss.zss.ui.au.in.SelectSheetCommand.process(SelectSheetCommand.java:87)
>>	at org.zkoss.zss.ui.Spreadsheet.service(Spreadsheet.java:4941)
>>	at org.zkoss.zk.ui.impl.DesktopImpl.service(DesktopImpl.java:708)
>>	at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1204)
>>	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)