Error causes program to keep running even after program close

Question

I am getting an error, and I am not sure what it means. I think it has something to do with threading. What I am thinking that is happening, is that when I start my thread it isn't stopping or something, because when I end my program the process keeps running. I can wrap the Surf.run() block in a try-catch to hide the error, but that doesn't solve the issue. What is causing the problem?

A method in my controller:

public void surf(){
    surf.addListener(new Runnable(){
        @Override
        public void run(){
            Browser.setPage(Surf.json.get("url").getAsString());
            timer.setText(Surf.json.get("wait_time").getAsString());
            waitTime = Surf.json.get("wait_time").getAsInt();
            timer.setVisible(true);
            startCounter();
        }
    });
    surf.run();
}

public class Surf implements Runnable{

    private java.util.List<Runnable> listeners = Collections.synchronizedList(new ArrayList<Runnable>());

    @Override
    public void run(){
        json = request();
        notifyListeners();
    }

    public void addListener(Runnable listener){
        listeners.add(listener);
    }

    public void removeListener(Runnable listener){
        listeners.remove(listener);
    }

    private void notifyListeners(){
        synchronized(listeners){
            for(Runnable listener : listeners){
                Platform.runLater(listener);
                this.removeListener(listener);
            }
        }
    }
}

Here is the Error:

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1768)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1651)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:204)
    at javafx.scene.Node.fireEvent(Node.java:8175)
    at javafx.scene.control.Button.fire(Button.java:185)
    at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:204)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3746)
    at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3471)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1695)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2486)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:314)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:243)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:345)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
    at com.sun.glass.ui.View.notifyMouse(View.java:898)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
    at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1765)
    ... 47 more
Caused by: java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:886)
    at java.util.ArrayList$Itr.next(ArrayList.java:836)
    at phantom.Surf.notifyListeners(Surf.java:66)
    at phantom.Surf.run(Surf.java:50)
    at phantom.FXMLDocumentController.surf(FXMLDocumentController.java:158)
    at phantom.FXMLDocumentController.beginSurfing(FXMLDocumentController.java:88)
    ... 57 more

Answer 1

assylias is correct, you need to use an Iterator to remove something from your collection as you are iterating through it. Here is an example for more illustration:

Iterator<Runnable> runIt = listeners.iterator();
while (runIt.hasNext())
{
    Runnable listener = runIt.next();
    Platform.runLater(listener);
    runIt.remove();
}

Answer 2

Your issue is in:

for(Runnable listener : listeners){
    Platform.runLater(listener);
    this.removeListener(listener);
}

You need to use an iterator if you want to remove from a collection while iterating.