Reproducing Steps:

1. run zss.test/display.zul
2. select cell-data sheet

Actual:
An exception is thrown.

2014/1/22 下午 02:54:45 org.zkoss.zk.ui.impl.UiEngineImpl handleError:1352
嚴重的: >>java.lang.NullPointerException
>> at org.zkoss.zss.model.impl.BookHelper.getFormatText(BookHelper.java:1009)
>> at org.zkoss.zss.model.impl.BookHelper.getCellText(BookHelper.java:1032)
>> at org.zkoss.zss.model.impl.BookHelper.getValidationList(BookHelper.java:4947)
>> at org.zkoss.zss.ui.Spreadsheet.convertDataValidationToJSON(Spreadsheet.java:1459)
>> at org.zkoss.zss.ui.Spreadsheet.convertDataValidationToJSON(Spreadsheet.java:1429)
>> at org.zkoss.zss.ui.Spreadsheet.updateSheetAttributes(Spreadsheet.java:828)
>> at org.zkoss.zss.ui.Spreadsheet.setSelectedSheetDirectly(Spreadsheet.java:769)
>> at org.zkoss.zss.ui.au.in.SelectSheetCommand.process(SelectSheetCommand.java:78)
>> at org.zkoss.zss.ui.Spreadsheet.service(Spreadsheet.java:4735)
>> at org.zkoss.zk.ui.impl.DesktopImpl.service(DesktopImpl.java:698)
>> 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)
>> 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)

Expected:
no error and display the sheet content.