ConcurrentModificationException -多线程

 new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                try {
                    Main.frame.onUpdate();
                    Thread.sleep(50);
                    for (Entity e : Main.frame.getEntityHandler().getEntities()) {
                        e.onUpdate();
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }).start();

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)
at de.crade.ki.logic.update.UpdateHandler$1.run(UpdateHandler.java:15)
at java.lang.Thread.run(Thread.java:745)