To reproduce.
1.There is a book series book1.xlsx, book2.xlsx, if book1 has cell(A1) refers to [book2.xlsx]Sheet1!B1
2.Use another book3.xlsx, and make it book-series with book2.xlsx (use the same book2 instance),
3.Then update book2-Sheet1!B1, you will get a NPE, it was cause by spreadsheet try to update book1-Sheet!A1 but it miss-updates to book3.

(Note, You shouldn't just avoid the check of null cell in bookhleper line 302, should remove the wrong dependence after reset book-series)

>>java.lang.NullPointerException
>> at org.zkoss.zss.model.sys.impl.BookHelper.reevaluate(BookHelper.java:302)
>> at org.zkoss.zss.model.sys.impl.BookHelper.reevaluateAndNotify(BookHelper.java:462)
>> at org.zkoss.zss.model.sys.impl.XRangeImpl.reevaluateAndNotify(XRangeImpl.java:434)
>> at org.zkoss.zss.model.sys.impl.XRangeImpl.setEditText(XRangeImpl.java:398)
>> at org.zkoss.zss.api.impl.RangeImpl.setCellEditText(RangeImpl.java:606)
>> at org.zkoss.zss.ui.Spreadsheet.processStopEditing0(Spreadsheet.java:4389)
>> at org.zkoss.zss.ui.Spreadsheet.processStopEditing(Spreadsheet.java:4352)
>> at org.zkoss.zss.ui.Spreadsheet.access$42(Spreadsheet.java:4350)
>> at org.zkoss.zss.ui.Spreadsheet$2.onEvent(Spreadsheet.java:319)
>> at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2748)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2719)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2660)
>> at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1717)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1502)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1212)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:600)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:482)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:491)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
>> at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
>> at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:401)
>> at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
>> at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
>> at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:766)
>> at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:450)
>> at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
>> at org.mortbay.jetty.Server.handle(Server.java:326)
>> at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
>> at org.mortbay.jetty.HttpConnection$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)