output: #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);
      }
input: #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