#vulnerable code 
                                public Node nextSibling() {
        if (parentNode == null)
            return null; // root
        
        List<Node> siblings = parentNode.childNodes;
        Integer index = indexInList(this, siblings);
        Validate.notNull(index);
        if (siblings.size() > index+1)
            return siblings.get(index+1);
        else
            return null;
    } 
                            #location 8 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public Node nextSibling() {
        if (parentNode == null)
            return null; // root
        
        List<Node> siblings = parentNode.childNodes;
        Integer index = siblingIndex();
        Validate.notNull(index);
        if (siblings.size() > index+1)
            return siblings.get(index+1);
        else
            return null;
    }