Exception in thread "Timer-0" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437)
at java.util.HashMap$EntryIterator.next(HashMap.java:1471)
at java.util.HashMap$EntryIterator.next(HashMap.java:1469)
at java.util.AbstractCollection.addAll(AbstractCollection.java:343)
at java.util.HashSet.<init>(HashSet.java:119)
at javafx.scene.CssStyleHelper.resetToInitialValues(CssStyleHelper.java:441)
at javafx.scene.CssStyleHelper.createStyleHelper(CssStyleHelper.java:180)
at javafx.scene.Node.reapplyCss(Node.java:8985)
at javafx.scene.Node.reapplyCss(Node.java:9014)
at javafx.scene.Node.impl_reapplyCSS(Node.java:8948)
at javafx.scene.Node$4.invalidated(Node.java:1110)
at javafx.beans.property.StringPropertyBase.markInvalid(StringPropertyBase.java:109)
at javafx.beans.property.StringPropertyBase.set(StringPropertyBase.java:144)
at javafx.scene.Node$4.set(Node.java:1103)
at javafx.scene.Node$4.set(Node.java:1097)
at javafx.scene.Node.setStyle(Node.java:1075)
at sample.Controller.updateTimerDisplay(Controller.java:66)
at sample.Pomodoro.handleUpdateStatus(Pomodoro.java:68)
at sample.Pomodoro.access$600(Pomodoro.java:8)
at sample.Pomodoro$1.run(Pomodoro.java:56)
at java.util.TimerThread.mainLoop(Timer.java:555)
at java.util.TimerThread.run(Timer.java:505)

public void start() {
    timer = new Timer();
    String soundFile = "./resources/sounds/boxing_bell.wav";
    status = State.FOCUSED;
    counter = 0;
    count = new TimerTask() {
        @Override
        public void run() {
            if(!paused) {
                counter++;
                displayTimer(controller.getTimer());
            }
            if(counter == maxSeconds) {
                if (status == State.FOCUSED) {
                    status = State.BREAK;
                    counter = 0;
                    maxSeconds = breakTime;
                    Sound sound = new Sound(soundFile);
                    sound.playSound();
                    handleUpdateStatus(status);
                } else {
                    status = State.FOCUSED;
                    counter = 0;
                    maxSeconds = focusTime;
                    Sound sound = new Sound(soundFile);
                    sound.playSound();
                    handleUpdateStatus(status);
                }
            }
        }
    };
    controller.updateTimerDisplay(Controller.Color.BLACK);
    timer.scheduleAtFixedRate(count, 1000,1000);
}