private void throwIfClosed() {
    if (parent.isClosed()) {
      LOG.error("Attempting to use a closed RocksDBKeyValueStorage");
      throw new StorageException("Storage has been closed");
    }
  }
Constructor which takes an explicit backing map for the layered key value storage.

@param map the backing map
@param parent the parent key value storage for this layered storage.
services/kvstore/src/main/java/org/hyperledger/besu/services/kvstore/LayeredKeyValueStorage.java