#vulnerable code 
                                public Document parse() {
        // TODO: figure out implicit head & body elements
        Document doc = new Document();
        stack.add(doc);

        while (tokenStream.hasNext()) {
            Token token = tokenStream.next();

            if (token.isStartTag()) {
                Attributes attributes = attributeParser.parse(token.getAttributeString());
                Tag tag = Tag.valueOf(token.getTagName());
                StartTag startTag = new StartTag(tag, attributes);

                // if tag is "html", we already have it, so OK to ignore skip. todo: abstract this. and can there be attributes to set?
                if (doc.getTag().equals(tag)) continue;

                Element parent = popStackToSuitableContainer(tag);
                Validate.notNull(parent, "Should always have a viable container");
                Element node = new Element(parent, startTag);
                parent.addChild(node);
                stack.add(node);
            }

            if (token.isEndTag()) { // empty tags are both start and end tags
                stack.removeLast();
            }

            // TODO[must] handle comments

            else if (token.isTextNode()) {
                String text = token.getData();
                TextNode textNode = new TextNode(stack.peek(), text);
                stack.getLast().addChild(textNode);
            }
        }
        return doc;
    } 
                            #location 20 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public Document parse() {
        // TODO: figure out implicit head & body elements
        Document doc = new Document();
        stack.add(doc);

        StringBuilder commentAccum = null;

        while (tokenStream.hasNext()) {
            Token token = tokenStream.next();

            if (token.isFullComment()) { // <!-- comment -->
                Comment comment = new Comment(stack.peek(), token.getCommentData());
                stack.getLast().addChild(comment);
            } else if (token.isStartComment()) { // <!-- comment
                commentAccum = new StringBuilder(token.getCommentData());
            } else if (token.isEndComment() && commentAccum != null) { // comment -->
                commentAccum.append(token.getCommentData());
                Comment comment = new Comment(stack.peek(), commentAccum.toString());
                stack.getLast().addChild(comment);
                commentAccum = null;
            } else if (commentAccum != null) { // within a comment
                commentAccum.append(token.getData());
            }

            else if (token.isStartTag()) {
                Attributes attributes = attributeParser.parse(token.getAttributeString());
                Tag tag = Tag.valueOf(token.getTagName());
                StartTag startTag = new StartTag(tag, attributes);

                // if tag is "html", we already have it, so OK to ignore skip. todo: abstract this. and can there be attributes to set?
                if (doc.getTag().equals(tag)) continue;

                Element parent = popStackToSuitableContainer(tag);
                Validate.notNull(parent, "Should always have a viable container");
                Element node = new Element(parent, startTag);
                parent.addChild(node);
                stack.add(node);
            }

            if (token.isEndTag() && commentAccum == null) { // empty tags are both start and end tags
                stack.removeLast();
            }



            // TODO[must] handle comments

            else if (token.isTextNode()) {
                String text = token.getData();
                TextNode textNode = new TextNode(stack.peek(), text);
                stack.getLast().addChild(textNode);
            }
        }
        return doc;
    }