从arraylist创建treeset时,什么导致了NPE?

WildFly Full 12.0.0.Final (WildFly Core 4.0.0.Final)
jdk1.8.0_121
[org.jboss.weld.Version] (MSC service thread 1-2) WELD-000900: 3.0.3 (Final)

public class KeywordPersist implements Serializable {

    private static final long serialVersionUID = 1L;

    private final String doi;
    private final List<String> keywords = new ArrayList<>();

    public KeywordPersist(final String doi, final List<String> keywords) {
        super();
        this.doi = doi;
        this.keywords.clear();
        this.keywords.addAll(keywords);
    }

//Getters, Setters, Hashcode, equals, toString NOT shown

}

我将它注入到无状态EJB中

@Inject
Event<KeywordPersist> keywordPersistEvent;

if ((result.getKeywordList() == null) || (result.getKeywordList().getKeyword() == null) || result.getKeywordList().getKeyword().isEmpty()) {
            return;
        }

        keywordPersistEvent.fire(generateKeywordEvent(result));

我生成事件数据如下:

private KeywordPersist generateKeywordEvent(final Result result) {
    final KeywordPersist keywordPersist = new KeywordPersist(result.getDoi(), new ArrayList<String>(result.getKeywordList().getKeyword()));
    return keywordPersist;
}

@Asynchronous
@Transactional(TxType.REQUIRES_NEW)
@Lock(LockType.WRITE)
public void parse(@Observes final KeywordPersist keywordPersist) {
    persistDoiKeyword(keywordPersist);
}

持久方法类似于:

/**
 * 
 * @param result
 */
private void persistDoiKeyword(final KeywordPersist keywordPersist) {

    if (keywordPersist == null) {
        System.out.println("---------------------------- persistDoiKeyword() if (keywordPersist == null) {}");
        throw new RuntimeException("Never can happen 0000");
    }

    if (keywordPersist.getDoi() == null) {
        System.out.println("---------------------------- persistDoiKeyword() if (keywordPersist.getDoi() == null) {}");
        throw new RuntimeException("Never can happen 0001");
    }

    if (keywordPersist.getKeywords() == null) {
        System.out.println("---------------------------- persistDoiKeyword() if (keywordPersist.getKeywords()== null) {}");
        throw new RuntimeException("Never can happen 0002");
    }

    if (keywordPersist.getKeywords().isEmpty()) {
        System.out.println("---------------------------- persistDoiKeyword() if (keywordPersist.getKeywords().isEmpty()) {}");
        throw new RuntimeException("Never can happen 0003");
    }

    final Set<String> uniqueKeywordsSet = new TreeSet<>(keywordPersist.getKeywords());
    final List<String> uniqueKeywords = new ArrayList<>(uniqueKeywordsSet);

    long rowCount = 0;

    for (String keyword : uniqueKeywords) {

        final DoiKeyword doiKeyword = new DoiKeyword();
        doiKeyword.setKeywordDoi(keywordPersist.getDoi());
        doiKeyword.setKeyword(cleanse(keyword));

        entityManager.persist(doiKeyword);

        rowCount++;

        if ((rowCount % 20) == 0) {
            entityManager.flush();
            entityManager.clear();
        }
    }

    entityManager.flush();
    entityManager.clear();
}

Caused by: java.lang.NullPointerException
    at java.util.TreeMap.put(TreeMap.java:563)
    at java.util.TreeSet.add(TreeSet.java:255)
    at java.util.AbstractCollection.addAll(AbstractCollection.java:344)
    at java.util.TreeSet.addAll(TreeSet.java:312)
    at java.util.TreeSet.<init>(TreeSet.java:160)
    at com.research.events.observers.KeywordPersistObserver.persistDoiKeyword(KeywordPersistObserver.java:66)
    at com.research.events.observers.KeywordPersistObserver.parse(KeywordPersistObserver.java:37)
    at sun.reflect.GeneratedMethodAccessor62.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.jboss.as.ee.component.ManagedReferenceMethodInterceptor.processInvocation(ManagedReferenceMethodInterceptor.java:52)
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:422)
    at org.jboss.invocation.InterceptorContext$Invocation.proceed(InterceptorContext.java:509)

这个NPE是从哪里来的?