[jxl] probleme pour dupliquer une feuille

Bonjour,

j'aimerais dupliquer une feuille d'un document Excel avec JXL.

Pour mes tests, cette feuille ne contient pour l'instant qu'une seule cellule renseignée par du texte (en position A1).
Je n'ai fais aucune mise en forme et j'aimerais copier celle-ci a la fin de mon classeur.

Pour ce faire, je créer mon workbook. Si je demande l'ouverture sans la copie tout se passe bien je retrouve bien ma valeur.
Mais, si j'ajoute :
workbook.copySheet(0, "copy", getNbSheets());
j'obtiens l'exception suivante :

Code:

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Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
      at jxl.write.biff.WritableSheetCopier.shallowCopyCells(WritableSheetCopier.java:499)
      at jxl.write.biff.WritableSheetCopier.copySheet(WritableSheetCopier.java:239)
      at jxl.write.biff.WritableSheetImpl.copy(WritableSheetImpl.java:1622)
      at jxl.write.biff.WritableWorkbookImpl.copySheet(WritableWorkbookImpl.java:987)
      at com.example.commons.excel.JExcelManager.duplicateSheet(JExcelManager.java:224)
      at com.example.vlab.ui.export.ExportMachineHCMTC2xls.exportResults(ExportMachineHCMTC2xls.java:59)
      at com.example.vlab.ui.FrmMainController.exportMachineStandardHCMTC(FrmMainController.java:214)
      at com.example.vlab.ui.FrmMainController.export(FrmMainController.java:470)
      at com.example.vlab.ui.FrmMain.fireExport(FrmMain.java:600)
      at com.example.vlab.ui.FrmMain.access$12(FrmMain.java:595)
      at com.example.vlab.ui.FrmMain$21.actionPerformed(FrmMain.java:437)
      at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
      at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
      at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
      at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
      at javax.swing.AbstractButton.doClick(Unknown Source)
      at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Unknown Source)
      at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(Unknown Source)
      at javax.swing.plaf.basic.BasicMenuItemUI$MouseInputHandler.mouseReleased(Unknown Source)
      at java.awt.Component.processMouseEvent(Unknown Source)
      at javax.swing.JComponent.processMouseEvent(Unknown Source)
      at java.awt.Component.processEvent(Unknown Source)
      at java.awt.Container.processEvent(Unknown Source)
      at java.awt.Component.dispatchEventImpl(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Window.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.EventQueue.dispatchEvent(Unknown Source)
      at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.run(Unknown Source)

Auriez-vous une idée pour résoudre ce probleme ?
Merci d'avance,
Noctis