#vulnerable code 
                                @Test
    public void outlinesTest() throws IOException, PdfException {
        PdfReader reader = new PdfReader(new FileInputStream(sourceFolder+"iphone_user_guide.pdf"));

        PdfDocument pdfDoc = new PdfDocument(reader);
        PdfOutline outlines = pdfDoc.getCatalog().getOutlines();
        List<PdfOutline> children = outlines.getAllChildren().get(0).getAllChildren();

        Assert.assertEquals(outlines.getTitle(), "Outlines");
        Assert.assertEquals(children.size(), 13);
        Assert.assertTrue(children.get(0).getDestination() instanceof PdfStringDestination);
    } 
                            #location 6 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void outlinesTest() throws IOException, PdfException {
        PdfReader reader = new PdfReader(new FileInputStream(sourceFolder+"iphone_user_guide.pdf"));

        PdfDocument pdfDoc = new PdfDocument(reader);
        PdfOutline outlines = pdfDoc.getOutlines(false);
        List<PdfOutline> children = outlines.getAllChildren().get(0).getAllChildren();

        Assert.assertEquals(outlines.getTitle(), "Outlines");
        Assert.assertEquals(children.size(), 13);
        Assert.assertTrue(children.get(0).getDestination() instanceof PdfStringDestination);
    }