Executor.submit() returns null

Question

I am working in a multithreaded environment with many methods to be called using Executors. The code works fine even for large data earlier, but it threw a null pointer exception once. Here is the related parts of the code.

private ExecutorService crawlerTaskExecutor = Executors.newFixedThreadPool(CRAWLER_THREAD_POOL_SIZE);
private List<Future<Boolean>> futureList = new ArrayList<>();

futureList.add(crawlerTaskExecutor.submit(() -> fetchDocuments(siteUrl),Boolean.FALSE));
//Many other method calls are also present.

private boolean isCrawlerRunning() {
    boolean isRunning = Boolean.FALSE;
    if (Objects.nonNull(futureList)) {
        for (Future<Boolean> future : futureList) {
            if (!future.isDone()) {//got null pointer here.
                isRunning = Boolean.TRUE;
                break;
            }
        }
    }
    return isRunning;
}

I got a null pointer in line if (!future.isDone()). Now I am not getting why would the future be null? I understand that crawlerTaskExecutor.submit is returning null but why and when it can return null? And what could be done if it returns null?

I am not getting why this happened, this issue doesn't get produced earlier even with lots of run with 500k data and suddenly appeared.