code: void closeIterator(LevelDBIterator<?> it) throws IOException {
      synchronized (this._db) {
        DB _db = this._db.get();
        if (_db != null) {
          it.close();
        }
      }
    }
docs: Closes the given iterator if the DB is still open. Trying to close a JNI LevelDB handle
  with a closed DB can cause JVM crashes, so this ensures that situation does not happen.