1. Open demo-sample.xlsx in zssapp
2. Press export to PDF
3. RuntimeException "The document is not open".

java.lang.RuntimeException: The document is not open.
at com.lowagie.text.pdf.PdfWriter.getDirectContent(Unknown Source)
at org.zkoss.zss.model.impl.pdf.PdfExporter.printPage(PdfExporter.java:1139)
at org.zkoss.zss.model.impl.pdf.PdfExporter.printSheet(PdfExporter.java:1068)
at org.zkoss.zss.model.impl.pdf.PdfExporter.exportSheetSelection(PdfExporter.java:690)
at org.zkoss.zss.model.impl.pdf.PdfExporter.exportSheet(PdfExporter.java:785)
at org.zkoss.zss.model.impl.pdf.PdfExporter.export(PdfExporter.java:258)
at org.zkoss.zss.api.impl.ExporterImpl.export(ExporterImpl.java:49)
at org.zkoss.zss.app.repository.impl.BookUtil.saveBookToWorkingFolder(BookUtil.java:133)
at org.zkoss.zss.app.ui.AppCtrl.doExportPdf(AppCtrl.java:349)
at org.zkoss.zss.app.ui.AppCtrl.onAppEvent(AppCtrl.java:375)
at org.zkoss.zss.app.ui.CtrlBase$1.onEvent(CtrlBase.java:64)
at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2752)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2723)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2664)
at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1765)
at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1550)
at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1260)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:603)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:485)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:494)
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)