NullPointerException

[Wollfram]

https://github.com/esin88/MessageSystem/blob/7b7545d3ce03258e76c4f39fd976110e3d776918/src/messageSystem/MessageSystem.java#L31-L34

if (!messageQueue.isEmpty()) {
// Two threads can come here and expect to receive message, while messageQueue.size()==1
messageQueue.pool().exec(abonent); // NullPointerException.
}