#vulnerable code 
                                public Node previousSibling() {
        List<Node> siblings = parentNode.childNodes;
        Integer index = indexInList(this, siblings);
        Validate.notNull(index);
        if (index > 0)
            return siblings.get(index-1);
        else
            return null;
    } 
                            #location 5 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public Node previousSibling() {
        List<Node> siblings = parentNode.childNodes;
        Integer index = siblingIndex();
        Validate.notNull(index);
        if (index > 0)
            return siblings.get(index-1);
        else
            return null;
    }