Run the following example app:

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.ButtonType;
import javafx.scene.control.Dialog;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.scene.control.TextArea;
import javafx.scene.input.KeyCombination;
import javafx.scene.layout.Priority;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class DialogConcurrentModification extends Application {

private class MyDialog extends Dialog<Void> {

public MyDialog() {
setTitle("Test");
getDialogPane().setContent(createPresentation());
getDialogPane().getButtonTypes().addAll(ButtonType.CLOSE);
}

private TextArea output = new TextArea() {
public void copy() {
super.copy();
close();
};
};

private Node createPresentation() {
VBox vbox = new VBox();
output.setText("Test 123");
output.setEditable(false);
output.prefWidthProperty().bind(vbox.widthProperty());
output.setPrefHeight(300);
output.selectAll();
VBox.setVgrow(output, Priority.ALWAYS);
vbox.getChildren().addAll(output);
return vbox;
}
}

public DialogConcurrentModification() {
}

@Override
public void start(Stage stage) throws Exception {
stage.setTitle("DialogConcurrentModification");
Scene scene = new Scene(new VBox(), 400, 350);

MenuBar menuBar = new MenuBar();

// --- Menu File
Menu menuFile = new Menu("File");
menuBar.getMenus().addAll(menuFile);

MenuItem test = new MenuItem("Open Dialog");
test.setAccelerator(KeyCombination.keyCombination("Ctrl+O"));
test.setOnAction(new EventHandler<ActionEvent>() {
public void handle(ActionEvent t) {
Dialog<Void> dlg = new MyDialog();
dlg.showAndWait();
}
});

menuFile.getItems().add(test);

((VBox) scene.getRoot()).getChildren().addAll(menuBar);

stage.setScene(scene);
stage.show();
}

public static void main(String[] args) {
launch(args);
}
}

Open the dialog (Ctrl+O), then right-click into the textarea and select "copy" from the context-menu. This should copy the text into the clipboard and close the dialog.

Under Windows (and only there) an Exception is thrown however:
Exception in thread "JavaFX Application Thread" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)
at com.sun.javafx.tk.quantum.GlassStage.windowsSetEnabled(GlassStage.java:166)
at com.sun.javafx.tk.quantum.WindowStage.setVisible(WindowStage.java:438)
at javafx.stage.Window$9.invalidated(Window.java:791)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:109)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:143)
at javafx.stage.Window.setShowing(Window.java:841)
at javafx.stage.Window.hide(Window.java:866)
at javafx.scene.control.HeavyweightDialog.close(HeavyweightDialog.java:205)
at javafx.scene.control.Dialog.close(Dialog.java:349)
at prv.rli.codetest.DialogConcurrentModification$MyDialog$1.copy(DialogConcurrentModification.java:32)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.callAction(TextInputControlBehavior.java:165)
at com.sun.javafx.scene.control.behavior.TextAreaBehavior.callAction(TextAreaBehavior.java:255)
at com.sun.javafx.scene.control.skin.TextInputControlSkin$ContextMenuItem.lambda$new$190(TextInputControlSkin.java:671)
at com.sun.javafx.scene.control.skin.TextInputControlSkin$ContextMenuItem$$Lambda$215/400489718.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1364)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.lambda$createChildren$324(ContextMenuContent.java:1317)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer$$Lambda$282/1216030339.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
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.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
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.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
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.Scene$MouseHandler.process(Scene.java:3724)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3452)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1728)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2461)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:348)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:273)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:382)
at com.sun.glass.ui.View.handleMouseEvent(View.java:553)
at com.sun.glass.ui.View.notifyMouse(View.java:925)
at com.sun.glass.ui.win.WinApplication._enterNestedEventLoopImpl(Native Method)
at com.sun.glass.ui.win.WinApplication._enterNestedEventLoop(WinApplication.java:129)
at com.sun.glass.ui.Application.enterNestedEventLoop(Application.java:513)
at com.sun.glass.ui.EventLoop.enter(EventLoop.java:107)
at com.sun.javafx.tk.quantum.QuantumToolkit.enterNestedEventLoop(QuantumToolkit.java:519)
at javafx.stage.Stage.showAndWait(Stage.java:462)
at javafx.scene.control.HeavyweightDialog.showAndWait(HeavyweightDialog.java:200)
at javafx.scene.control.Dialog.showAndWait(Dialog.java:295)
at prv.rli.codetest.DialogConcurrentModification$1.handle(DialogConcurrentModification.java:68)
at prv.rli.codetest.DialogConcurrentModification$1.handle(DialogConcurrentModification.java:1)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.ControlAcceleratorSupport.lambda$doAcceleratorInstall$12(ControlAcceleratorSupport.java:163)
at com.sun.javafx.scene.control.ControlAcceleratorSupport$$Lambda$119/148261540.run(Unknown Source)
at com.sun.javafx.scene.KeyboardShortcutsHandler.processAccelerators(KeyboardShortcutsHandler.java:348)
at com.sun.javafx.scene.KeyboardShortcutsHandler.dispatchBubblingEvent(KeyboardShortcutsHandler.java:166)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$KeyHandler.process(Scene.java:3931)
at javafx.scene.Scene$KeyHandler.access$1800(Scene.java:3877)
at javafx.scene.Scene.impl_processKeyEvent(Scene.java:2006)
at javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2468)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:197)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:147)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:227)
at com.sun.glass.ui.View.handleKeyEvent(View.java:544)
at com.sun.glass.ui.View.notifyKey(View.java:954)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$141(WinApplication.java:102)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/1109371569.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)