Reproducing Steps:

1. open zss.test/display2003.zul
2. delete sheet ""chart-image""
3. select the sheet "chart-unsupported"

Actual:
a exception is trown.

警告: fail to load chart series data
java.lang.NullPointerException
at org.zkoss.zss.model.sys.impl.XRangeImpl.initCellRef(XRangeImpl.java:95)
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.util.ChartHelper.prepareTitle(ChartHelper.java:478)
at org.zkoss.zssex.util.ChartHelper.prepareXYModel(ChartHelper.java:403)
at org.zkoss.zssex.util.ChartHelper.prepareScatterModel(ChartHelper.java:361)
at org.zkoss.zssex.util.ChartHelper.drawHSSFChart(ChartHelper.java:351)
at org.zkoss.zssex.util.ChartHelper.drawChart(ChartHelper.java:324)
at org.zkoss.zssex.ui.widget.ChartWidget.initChart(ChartWidget.java:196)
at org.zkoss.zssex.ui.widget.ChartWidget.<init>(ChartWidget.java:57)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.newChartWidget(DefaultBookWidgetLoader.java:422)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.newChartWidgetGroup(DefaultBookWidgetLoader.java:415)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.prepareChartWidgets(DefaultBookWidgetLoader.java:186)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.onSheetSelected(DefaultBookWidgetLoader.java:93)
at org.zkoss.zss.ui.Spreadsheet.afterSheetSelected(Spreadsheet.java:4184)
at org.zkoss.zss.ui.Spreadsheet.setSelectedSheet(Spreadsheet.java:833)
at org.zkoss.zss.ui.impl.ua.DeleteSheetHandler.processAction(DeleteSheetHandler.java:62)
at org.zkoss.zss.ui.impl.ua.AbstractHandler.process(AbstractHandler.java:57)
at org.zkoss.zss.ui.impl.DefaultUserActionManagerCtrl.dispatchAuxAction(DefaultUserActionManagerCtrl.java:272)
at org.zkoss.zss.ui.impl.DefaultUserActionManagerCtrl.onEvent(DefaultUserActionManagerCtrl.java:395)
at org.zkoss.zssex.ui.impl.UserActionManagerCtrlImpl.onEvent(UserActionManagerCtrlImpl.java:288)
at org.zkoss.zss.ui.Spreadsheet$3.onEvent(Spreadsheet.java:388)
at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2743)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2714)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2655)
at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1720)
at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1505)
at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1215)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:601)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:483)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:492)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:401)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:766)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:450)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$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)