Steps to reproduce:

- Create a JavaFX Script file
----------------------------------------------
import javafx.scene.*;
import javafx.scene.paint.*;
import javafx.application.*;

Frame {
    title: "MyApplication"
    width: 200
    height: 200
    closeAction: function() {
        java.lang.System.exit( 0 );
    }
    visible: true

    scene: Scene {
        fill: Color.BLACK
        content: []
    }
}
----------------------------------------------

- Enable Preview
- Close the Design Preview window

  There is an NPE in output window:

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:591)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:284)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)