EDIT

import com.vaadin.ui.UI;

import com.vaadin.ui.Window;

        cancel.addClickListener(e -> cancelEditorDialog());

    protected void cancelEditorDialog(){

        if(fieldGroup.isModified()){

            String message = "<p>The editor has been modified.<br>Do you want to save your changes or discard them?<p>";

            Window editorModifiedDialog = new Window("Cancel editor");

            editorModifiedDialog.setModal(true);

            VerticalLayout subContent = new VerticalLayout();

            subContent.setMargin(true);

            editorModifiedDialog.setContent(subContent);

            Button save = new Button("Save", FontAwesome.SAVE);

            save.setStyleName(ValoTheme.BUTTON_PRIMARY);

            Button discard = new Button("Discard", FontAwesome.REMOVE);

            discard.setStyleName(ValoTheme.BUTTON_DANGER);

            Button cancel = new Button("Cancel", FontAwesome.ARROW_CIRCLE_LEFT);

            Label messageLabel = new Label(message, com.vaadin.shared.ui.label.ContentMode.HTML);

            subContent.addComponent(messageLabel);

            HorizontalLayout buttonBar = new HorizontalLayout(save, discard, cancel);

            buttonBar.setStyleName(ValoTheme.WINDOW_BOTTOM_TOOLBAR);

            buttonBar.setWidth(100, Unit.PERCENTAGE);

            buttonBar.setSpacing(true);

            buttonBar.setExpandRatio(save, 1);

            buttonBar.setComponentAlignment(discard, Alignment.TOP_RIGHT);

            buttonBar.setComponentAlignment(save, Alignment.TOP_RIGHT);

            buttonBar.setComponentAlignment(cancel, Alignment.TOP_RIGHT);

            subContent.addComponent(buttonBar);

            save.addClickListener(e -> {editorModifiedDialog.close(); save();});

            discard.addClickListener(e -> {editorModifiedDialog.close(); cancel();});

            cancel.addClickListener(e -> editorModifiedDialog.close());

            UI.getCurrent().addWindow(editorModifiedDialog);

        } else {

            cancel();

        }

    }