Select menu item Preview > Scene Style Sheets > Add a Style Sheet…
Select Cancel to return from File Chooser without selecting a file.

This causes the following NPE:
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at com.oracle.javafx.scenebuilder.app.SceneStyleSheetMenuController.performAddSceneStyleSheet(SceneStyleSheetMenuController.java:68)
at com.oracle.javafx.scenebuilder.app.DocumentWindowController.performControlAction(DocumentWindowController.java:403)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$DocumentControlActionController.perform(MenuBarController.java:1146)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.handleOnActionMenu(MenuBarController.java:784)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.access$700(MenuBarController.java:81)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$10.handle(MenuBarController.java:777)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$10.handle(MenuBarController.java:773)
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.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
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.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.GlobalMenuAdapter$5.handle(GlobalMenuAdapter.java:158)
at com.sun.javafx.scene.control.GlobalMenuAdapter$5.handle(GlobalMenuAdapter.java:156)
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.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
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.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.tk.quantum.GlassSystemMenu$6.action(GlassSystemMenu.java:234)