code: @PostConstruct
      public void init() {
          try {
              popup = new JFXPopup(FXMLLoader.load(getClass().getResource("/fxml/ui/popup/DemoPopup.fxml")));
          } catch (IOException ioExc) {
              ioExc.printStackTrace();
          }
          burger1.setOnMouseClicked((e) -> popup.show(rippler1, PopupVPosition.TOP, PopupHPosition.LEFT));
          burger2.setOnMouseClicked((e) -> popup.show(rippler2, PopupVPosition.TOP, PopupHPosition.RIGHT));
          burger3.setOnMouseClicked((e) -> popup.show(rippler3, PopupVPosition.BOTTOM, PopupHPosition.LEFT));
          burger4.setOnMouseClicked((e) -> popup.show(rippler4, PopupVPosition.BOTTOM, PopupHPosition.RIGHT));
      }
docs: init fxml when loaded.