Steps to reproduce:

- Create a JavaFX Script file:
------------------------------------------
import javafx.ext.swing.*;
import javafx.scene.geometry.*;
import javafx.scene.paint.*;

Frame{
    title: "MyFrame"
    content: Canvas{
        content: Circle{
            radius: 100
        }
    }
    visible: true
}
------------------------------------------
- Enable the Preview
- Disable the Preview

The exception pops up:

ava.lang.IllegalMonitorStateException
	at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:127)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1175)
	at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:431)
	at java.util.concurrent.DelayQueue.take(DelayQueue.java:176)
	at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:583)
	at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:576)
	at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:947)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
[catch] at java.lang.Thread.run(Thread.java:619)