java.util.ConcurrentModificationException in javafx timeline

Question

I haven't made Thread myself. I have one timeline that runs from the beginning to the end of the program which is as follows:

Timeline timeline = new Timeline(
        new KeyFrame(Duration.millis(TIMELINE_DELAY), event -> {
            intrudersList.forEach(Intruder::action);
            towersList.forEach(Tower::action);
            otherActivesList.forEach(Active::action);
        }));
timeline.setCycleCount(Timeline.INDEFINITE);
timeline.play();

And when the die method of the Intruder class is called, i get this Concurrent Modification Exception.

First, I don't actually understand how timeline works! Does it create new Threads itself or what? and what will happen if for example we have a timeline that does a task every 10 seconds and that task takes 15 seconds to be done! And second indeed: How can i get this fixed!?

public void die() {
    this.getCell().getContent().remove(this);
    TimeLine.getInstance().removeIntruder(this);
    System.out.println("death of intruder at: " + cell);
}

Answer 1

Many thanks to @DVarga, This is what the problem was and how i fixed it: the problem was that I was modifying the intrudersList in removeIntruder , while iterating on it in KeyFrame. And I solved it by wrapping the body of the removeIntruder method in a Platform.runLater(() -> { ... }) block. I Guess what Platform.runLater does is that it waits until the list is modifyable and then modifies it.