Code should be
SolrInputDocument document = new SolrInputDocument();
List<SolrInputDocument> additionalDocuments = _solrTrashElementIndexer.indexTrashElement(trashElement, document);
List<SolrInputDocument> documents = new ArrayList<>(additionalDocuments);
documents.add(document);
UpdateResponse solrResponse = _solrClient.add(_solrClientProvider.getCollectionName(_workspaceName), documents);