I get the following exception using Scene Builder,
when changing the Theme from Modena to Caspian.

To reproduce:
Open the attached fxml from SB.
Preview > JavaFX Themes > Caspian

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
at java.util.HashMap$EntryIterator.next(HashMap.java:1463)
at java.util.HashMap$EntryIterator.next(HashMap.java:1461)
at com.sun.javafx.css.StyleManager.forget(StyleManager.java:628)
at javafx.scene.SubScene$6.invalidated(SubScene.java:587)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:111)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
at javafx.scene.SubScene.setUserAgentStylesheet(SubScene.java:625)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.WorkspaceController.setThemeStyleSheet(WorkspaceController.java:154)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.ContentPanelController.themeDidChange(ContentPanelController.java:1022)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.ContentPanelController.lambda$3(ContentPanelController.java:185)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.ContentPanelController$$Lambda$109/289915814.changed(Unknown Source)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:361)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:81)
at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(ObjectPropertyBase.java:105)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
at javafx.beans.property.ObjectProperty.setValue(ObjectProperty.java:69)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.setTheme(EditorController.java:546)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$SetThemeActionController.perform(MenuBarController.java:2114)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.handleOnActionMenu(MenuBarController.java:1105)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.lambda$1(MenuBarController.java:1099)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$$Lambda$67/1975834308.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.GlobalMenuAdapter.lambda$bindMenuItemProperties$20(GlobalMenuAdapter.java:153)
at com.sun.javafx.scene.control.GlobalMenuAdapter$$Lambda$315/630508149.handle(Unknown Source)
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:198)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.tk.quantum.GlassSystemMenu$1.action(GlassSystemMenu.java:234)