Java SE (Java Platform, Standard Edition)

I wonder whether I picked the right forum here, if not, excuse me...

I'm working with NetBeans 6.1 under Mac OS X 10.5.5. (JDK 6) and am building a desktop application. Sometimes, when I have time consuming tasks to do, I simply open a modal JDialog with a progressbar and animated busy-icon (based on the basic stuff Netbeans automatically gives you when choosing a desktop app as a new project type) and start the background task manually in the constructor. This, for instance, looks like this:

    public CSaveDialog(java.awt.Frame parent, CDaten d) {
        super(parent);
        dataObj = d;        
        initComponents();

        // init the progress bar and status icon for
        // the swingworker background thread
        // creates a new class object. This variable is not used, it just associates task monitors to
        // the background tasks. furthermore, by doing this, this class object also animates the 
        // busy icon and the progress bar of this frame.
        CInitStatusBar isb = new CInitStatusBar(statusAnimationLabel, progressBar);
        // show status text
        msgLabel.setText(resourceMap.getString("msg1"));

        // start the background task manually
        Task svT = saveFile();
        // get the application's context...
	ApplicationContext appC = Application.getInstance().getContext();
        // ...to get the TaskMonitor and TaskService
        TaskMonitor tM = appC.getTaskMonitor();
        TaskService tS = appC.getTaskService();
        // with these we can execute the task and bring it to the foreground
        // i.e. making the animated progressbar and busy icon visible
        tS.execute(svT);
        tM.setForegroundTask(svT);
    }

The background task itself looks like this:

    @Action
    public Task saveFile() {
        return new SaveFileTask(org.jdesktop.application.Application.getInstance(zettelkasten.ZettelkastenApp.class));
    }

    private class SaveFileTask extends org.jdesktop.application.Task<Object, Void> {
        SaveFileTask(org.jdesktop.application.Application app) {
            // Runs on the EDT.  Copy GUI state that
            // doInBackground() depends on from parameters
            // to ImportFileTask fields, here.
            super(app);
            savingTask = this;
        }
        @Override
        protected Object doInBackground() throws IOException {
            // Your Task's code here.  This method runs
            // on a background thread, so don't reference
            // the Swing GUI from here.
            // prevent task from processing when the file path is incorrect
            
            try {
                ZipOutputStream zip = new ZipOutputStream(new FileOutputStream(fp));
                XMLOutputter out = new XMLOutputter();

                zip.putNextEntry(new ZipEntry("zknFile.xml"));
                out.output(dataObj.getZknData(), zip);

                zip.putNextEntry(new ZipEntry("authorFile.xml"));
                out.output(dataObj.getAuthorData(), zip);

                zip.putNextEntry(new ZipEntry("keywordFile.xml"));
                out.output(dataObj.getKeywordData(), zip);

                zip.close();
            }
            catch (IOException e) {
                e.printStackTrace();
                Logger.getLogger(CLoadDialog.class.getName()).log(Level.SEVERE, null, e);
            }            

            return null;  // return your result
        }
        @Override
        protected void succeeded(Object result) {
            // Runs on the EDT.  Update the GUI based on
            // the result computed by doInBackground().
        }
        @Override
        protected void finished() {
            super.finished();
            // when the task is finished, clear it
            savingTask = null;
            // and close window
            dispose();
            setVisible(false);
        }
    }

In this way I have created many JDialogs, so it's all a simple copy'n'paste thing, but only in this case I get an error message, and I don't know why - it might be because I cannot really "decode" the dubug-log and find out what it's telling me. Here the error-message:

26.10.2008 17:24:51 org.jdesktop.application.Task failed
SCHWERWIEGEND: zettelkasten.CSaveDialog$SaveFileTask@bec15c7 failed: java.lang.NullPointerException
java.lang.NullPointerException
        at java.io.FileOutputStream.<init>(FileOutputStream.java:172)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:131)
        at zettelkasten.CSaveDialog$SaveFileTask.doInBackground(CSaveDialog.java:264)
        at org.jdesktop.swingworker.SwingWorker$1.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
        at java.util.concurrent.FutureTask.run(FutureTask.java:138)
        at org.jdesktop.swingworker.SwingWorker.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
        at java.lang.Thread.run(Thread.java:637)
BUILD SUCCESSFUL (total time: 33 seconds)

The curious thing is, that when I execute the code within the above shown "doInBackground"-method outside a background task, everything works well. Only when I put the method to save XML files and store them in a ZIP file into a background task, the error occurs - although I simply copied'n'pasted the code.

Btw, this is how I open the dialog. I copied this from the "ShowAboutBox", which is a pre-generated source-part of Netbeans:

        // opens the Import Dialog. This Class is responsible
        // for getting the relevant import data. the import task
        // itself (background task) will be started as another dialog,
        // when this one is closed
        JFrame mainFrame = ZettelkastenApp.getApplication().getMainFrame();
        // if dialog window isn't already created, do this now
        if (null == saveDlg) {
            // get parent und init window
            saveDlg = new CSaveDialog(mainFrame,data);
            // center window
            saveDlg.setLocationRelativeTo(mainFrame);
        }
        ZettelkastenApp.getApplication().show(saveDlg); 

        saveDlg.dispose();
        saveDlg = null;

Does anyone have an idea?

Thanks in advance
Daniel