ADDITIONAL SYSTEM INFORMATION :
JavaFX 24.0.2

Java:
openjdk version "25.0.1" 2025-10-21 LTS
OpenJDK Runtime Environment Temurin-25.0.1+8 (build 25.0.1+8-LTS)
OpenJDK 64-Bit Server VM Temurin-25.0.1+8 (build 25.0.1+8-LTS, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
A NullPointerException is thrown when a submenu of a ContextMenu is opened for the second time, and the graphic and style classes are changed while the menu is open.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1: start the supplied reproducer program
2: open the ContextMenu or the main menu
3: hover the mouse over the 'menu...' item (this opens the submenu)
4: hover the mouse over the 'dummy' item (this hides the submenu)
5: hover the mouse over the 'menu...' item (this opens the submenu again) >>>>>> ERROR

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.application.Platform;
import javafx.scene.Scene;
import javafx.scene.control.ContextMenu;
import javafx.scene.control.Label;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class SubMenuNullPointer extends Application {

    public static class Launcher {
        public static void main(String[] args) {
            SubMenuNullPointer.main(args);
        }
    }

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        MenuBar menuBar = new MenuBar(new Menu("Main menu", null, new MenuItem("dummy"), createSubMenu()));

        ContextMenu contextMenu = new ContextMenu();
        contextMenu.getItems().addAll(new MenuItem("dummy"), createSubMenu());

        BorderPane root = new BorderPane();

        root.setTop(menuBar);
        Label label = new Label("open context menu here");
        label.setContextMenu(contextMenu);
        root.setCenter(label);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }

    private Menu createSubMenu() {
        Menu menu = new Menu("menu...");

        MenuItem item = new MenuItem("item");
        menu.getItems().add(item);

        menu.setOnShown(event -> {
            Platform.runLater(() -> {
                // Update the graphic while the menu is visible
                item.setGraphic(new Label("graphic"));

                // BUG: NullPointer
                item.getStyleClass().removeAll("test");
                item.getStyleClass().add("test");
            });
        });
        return menu;
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use a container node (e.g. StackPane) to wrap the MenuItem graphic and replace the graphic in the container node, rather than in the MenuItem itself.

FREQUENCY :
ALWAYS