#vulnerable code 
                              public Key<?> exists(final Object entityOrKey) {
    final Object unwrapped = ProxyHelper.unwrap(entityOrKey);
    final Key<?> key = mapper.getKey(unwrapped);
    final Object id = key.getId();
    if (id == null) {
      throw new MappingException("Could not get id for " + unwrapped.getClass().getName());
    }

    String collName = key.getKind();
    if (collName == null) {
      collName = getCollection(key.getKindClass()).getName();
    }

    return find(collName, key.getKindClass()).filter(Mapper.ID_KEY, key.getId()).getKey();
  } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              public Key<?> exists(final Object entityOrKey) {
    final Query<?> query = buildExistsQuery(entityOrKey);
    return query.getKey();
  }