Merge "Check if priority queue is empty before attempting to get first element" into main

    }

    private static Message first(ConcurrentSkipListSet<Message> queue) {

        // If the queue is empty, avoid calling queue.first() which will allocate

        // an exception that we'll immediately ignore.

        // We might race with another thread that's removing from the queue and

        // end up with the exception anyway, but at least we tried.

        if (queue.isEmpty()) {

            return null;

        }

        try {

            return queue.first();

        } catch (NoSuchElementException e) {