Expand bookmark to show child bookmarks in PDF file

Using the Aspose.PDF for Java (aspose-pdf-9.5.2-jdk16.jar), how can I create a bookmark so that it will already be expanded to show child bookmarks when the PDF file is opened in Adobe Reader and the bookmark view is opened? See sample code below. It creates all the bookmarks, but when the Bookmark view is opened, the "Document Bookmarks" is not expanded by default. Thanks

Bookmark topLevelBookmark = new Bookmark();
topLevelBookmark.setTitle("Document Bookmarks");
topLevelBookmark.setOpen(true);
topLevelBookmark.setPageNumber(1);

Bookmarks docBookmarks = new Bookmarks();

Bookmark docBookmark = new Bookmark();
docBookmark.setTitle("Sub Bookmark 1");
docBookmark.setOpen(true);
docBookmark.setPageNumber(1);
docBookmarks.add(docBookmark);
	
docBookmark = new Bookmark();
docBookmark.setTitle("Sub Bookmark 2");
docBookmark.setOpen(true);
docBookmark.setPageNumber(2);
docBookmarks.add(docBookmark);

topLevelBookmark.setChildItems(docBookmarks);

PdfBookmarkEditor pdfBookmarkEditor = new PdfBookmarkEditor();
pdfBookmarkEditor.bindPdf(combinedDocsPdfFileLocation);
pdfBookmarkEditor.deleteBookmarks();
pdfBookmarkEditor.createBookmarks(topLevelBookmark);
pdfBookmarkEditor.save(combinedDocsPdfFileLocation);
pdfBookmarkEditor.close();