public final WorkbenchDialogBuilder onHidden(EventHandler<Event> onHidden) {
      this.onHidden = onHidden;
      return this;
    }
The dialog's action, which is invoked whenever the dialog has been hidden in the scene graph.
An event will be fired whenever {@link DialogControl#hide()} or
{@link Workbench#hideDialog(WorkbenchDialog)} has been called or the dialog has been closed
by clicking on its corresponding {@link GlassPane}.

@param onHidden action to be performed
@return builder for chaining
workbenchfx-core/src/main/java/com/dlsc/workbenchfx/model/WorkbenchDialog.java