Reproducing Steps:

1. open zss.test/display2003.zul
2. move left the sheet ""chart-image""
3. select arbitrary another sheet, e.g. the sheet ""column""
4. select the sheet ""chart-image"" again

Expected:
the sheet ""chart-image"" showed

Actual:

嚴重的: >>java.lang.NullPointerException
>> at org.zkoss.zssex.util.ChartHelper.prepareTitle(ChartHelper.java:479)
>> at org.zkoss.zssex.util.ChartHelper.getFirstSeriesTitle(ChartHelper.java:461)
>> at org.zkoss.zssex.util.ChartHelper.getChartTitle(ChartHelper.java:451)
>> at org.zkoss.zssex.util.ChartHelper.drawHSSFChartInner(ChartHelper.java:441)
>> at org.zkoss.zssex.util.ChartHelper.drawHSSFChart(ChartHelper.java:356)
>> at org.zkoss.zssex.util.ChartHelper.drawChart(ChartHelper.java:324)
>> at org.zkoss.zssex.ui.widget.ChartWidget.initChart(ChartWidget.java:185)
>> 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.setSelectedSheetDirectly(Spreadsheet.java:877)
>> 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$RequestHandler.content(HttpConnection.java:945)
>> at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
>> at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
>> at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
>> at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410)
>> at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)