#vulnerable code 
                                protected void replaceChild(Node out, Node in) {
        Validate.isTrue(out.parentNode == this);
        Validate.notNull(in);
        if (in.parentNode != null)
            in.parentNode.removeChild(in);
        
        Integer index = indexInList(out, childNodes);
        childNodes.set(index, in);
        in.parentNode = this;
        out.parentNode = null;
    } 
                            #location 8 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                protected void replaceChild(Node out, Node in) {
        Validate.isTrue(out.parentNode == this);
        Validate.notNull(in);
        if (in.parentNode != null)
            in.parentNode.removeChild(in);
        
        Integer index = out.siblingIndex();
        childNodes.set(index, in);
        in.parentNode = this;
        in.setSiblingIndex(index);
        out.parentNode = null;
    }