Steps to reproduce

1. load attached file

Actual result

java.lang.NullPointerException
at org.zkoss.poi.ss.formula.ptg.ExternSheetNameResolver.prependSheetName(ExternSheetNameResolver.java:41)
at org.zkoss.poi.ss.formula.ptg.Area3DPtg.toFormulaString(Area3DPtg.java:100)
at org.zkoss.poi.ss.formula.FormulaRenderer.toFormulaString(FormulaRenderer.java:102)
at org.zkoss.poi.hssf.model.HSSFFormulaParser.toFormulaString(HSSFFormulaParser.java:83)
at org.zkoss.poi.hssf.usermodel.HSSFName.getRefersToFormula(HSSFName.java:200)
at org.zkoss.zss.range.impl.imexp.AbstractExcelImporter.skipName(AbstractExcelImporter.java:234)
at org.zkoss.zss.range.impl.imexp.ExcelXlsImporter.skipName(ExcelXlsImporter.java:393)
at org.zkoss.zss.range.impl.imexp.AbstractExcelImporter.importNamedRange(AbstractExcelImporter.java:211)
at org.zkoss.zss.range.impl.imexp.AbstractExcelImporter.imports(AbstractExcelImporter.java:149)
at org.zkoss.zss.range.impl.imexp.ExcelImportAdapter.imports(ExcelImportAdapter.java:46)
at org.zkoss.zss.range.impl.imexp.AbstractImporter.imports(AbstractImporter.java:54)
at org.zkoss.zss.ui.Spreadsheet.getSBook(Spreadsheet.java:642)
at org.zkoss.zss.ui.Spreadsheet.getSelectedSSheet(Spreadsheet.java:839)
at org.zkoss.zss.ui.Spreadsheet.doInvalidate(Spreadsheet.java:4662)
at org.zkoss.zss.ui.Spreadsheet.invalidate(Spreadsheet.java:4001)
at org.zkoss.zss.ui.Spreadsheet.setSrc(Spreadsheet.java:879)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.zkoss.zk.ui.metainfo.Property.assign0(Property.java:284)
at org.zkoss.zk.ui.metainfo.Property.assign(Property.java:169)
at org.zkoss.zk.ui.metainfo.ComponentInfo.applyProperties(ComponentInfo.java:729)
at org.zkoss.zk.ui.impl.AbstractUiFactory.newComponent(AbstractUiFactory.java:106)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:857)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:826)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:735)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:797)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:757)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:699)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:442)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:356)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:217)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:136)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
...

Proposed Spec

Ignore such invalid named ranges, log warning message with related information.