Estou tentando entender a API Stanford CoreNLP. Desejo que uma frase simples seja tokenizada usando o seguinte código:

    Properties props = new Properties();
    props.put("annotators", "tokenize");
    StanfordCoreNLP pipeline = new StanfordCoreNLP(props);

    // read some text in the text variable
    String text = "I wish this code would run.";

    // create an empty Annotation just with the given text
    Annotation document = new Annotation(text);

    // run all Annotators on this text
    pipeline.annotate(document);

    // these are all the sentences in this document
    // a CoreMap is essentially a Map that uses class objects as keys and has values with custom types
    List<CoreMap> sentences = document.get(SentencesAnnotation.class);

    for(CoreMap sentence: sentences) {
        // traversing the words in the current sentence
        // a CoreLabel is a CoreMap with additional token-specific methods
        for (CoreLabel token: sentence.get(TokensAnnotation.class)) {
            // this is the text of the token
            String word = token.get(TextAnnotation.class);
            // this is the POS tag of the token
            String pos = token.get(PartOfSpeechAnnotation.class);
            // this is the NER label of the token
            String ne = token.get(NamedEntityTagAnnotation.class);       
        }

        // this is the parse tree of the current sentence
        Tree tree = sentence.get(TreeAnnotation.class);

        // this is the Stanford dependency graph of the current sentence
        SemanticGraph dependencies = sentence.get(CollapsedCCProcessedDependenciesAnnotation.class);
    }

    // This is the coreference link graph
    // Each chain stores a set of mentions that link to each other,
    // along with a method for getting the most representative mention
    // Both sentence and token offsets start at 1!
    Map<Integer, CorefChain> graph = document.get(CorefChainAnnotation.class);

Isso foi retirado do próprio site da PNL de Stanford, então eu esperava que funcionasse imediatamente. Infelizmente isso não acontece, pois me fornece uma NullPointerException em:

for(CoreMap sentence: sentences) {...