public final Optional<Button> getButton(ButtonType buttonType) {
    return Optional.ofNullable(buttonNodes.get(buttonType));
  }
Retrieves the {@link Button} instance of the {@link DialogControl} which is of the specified
{@link ButtonType}.

@param buttonType to retrieve from the {@link DialogControl}
@return the button or an empty {@link Optional}, if the {@link DialogControl} hasn't been
    initialized before
workbenchfx-core/src/main/java/com/dlsc/workbenchfx/view/controls/dialog/DialogControl.java