#vulnerable code 
                                @Override
    public V remove(Object key) {
        V value = get(key);
        value.delete();
        nest.hdel(key.toString());
        return value;
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public V remove(Object key) {
        V value = get(key);
        nest.hdel(key.toString());
        return value;
    }