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  1. JDK
  2. JDK-8244234

MenuButton: NPE on removing from scene with open popup





        Bubbled up on SO https://stackoverflow.com/a/61539677/203657 - copied the example here for convenience.

        To reproduce, run open popup and click on menuItem
        - expected: new root showing without errors
        - actual: new root showing, NPE in MenuButtonSkin

        Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
            at java.base/java.util.Objects.requireNonNull(Objects.java:222)
            at javafx.controls/javafx.scene.control.skin.MenuButtonSkinBase.lambda$new$7(MenuButtonSkinBase.java:198)
            at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
            at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
            at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
            at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
            at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
            at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
            at java.base/java.lang.Thread.run(Thread.java:832)

        The example:

        import javafx.application.Application;
        import javafx.scene.Scene;
        import javafx.scene.control.Label;
        import javafx.scene.control.MenuButton;
        import javafx.scene.control.MenuItem;
        import javafx.scene.layout.StackPane;
        import javafx.stage.Stage;

        public class Main extends Application {

          public void start(Stage primaryStage) {
            MenuButton btn = new MenuButton("Switch...");

            MenuItem item = new MenuItem("To 'Hello, World!'");
                e -> {
                  StackPane root = new StackPane(new Label("Hello, World!"));

            primaryStage.setScene(new Scene(new StackPane(btn), 500, 300));

        Converted into a unit test:

            public void testMenuButtonSkinNPEOnReplaceSceneOpenPopup() {
                MenuButton menuButton = new MenuButton("switch");
                MenuItem menuItem = new MenuItem("Press '_a'");

                Scene scene = new Scene(menuButton);
                Stage stage = new Stage();
                menuItem.setOnAction(e -> {
                    StackPane root = new StackPane(new Label("Hello, World!"));
                assertTrue("sanity:", menuButton.isShowing());
                assertSame(scene.getFocusOwner(), menuButton);
                // trying to see the NPE
                // working options (the exact sequence of events is slightly different)
                // keyboard on menuItem and press accelerator
                KeyEventFirer keyboard = new KeyEventFirer(menuItem, scene);
                keyboard.doKeyPress(KeyCode.A, KeyModifier.ALT);
        The error happens in the showingListener installed in skin:

                    if (popup.isShowing()) {
                        boolean showMnemonics = NodeHelper.isShowMnemonics(getSkinnable());
                        Utils.addMnemonics(popup, getSkinnable().getScene(), showMnemonics, mnemonics);
                    } else {
                        // we wrap this in a runLater so that mnemonics are not removed
                        // before all key events are fired (because KEY_PRESSED might have
                        // been used to hide the menu, but KEY_TYPED and KEY_RELEASED
                        // events are still to be fired, and shouldn't miss out on going
                        // through the mnemonics code (especially in case they should be
                        // consumed to prevent them being used elsewhere).
                        // See JDK-8090026 for more detail.
        >>>> the scene here might be null
                        Scene scene = getSkinnable().getScene();
                        List<Mnemonic> mnemonicsToRemove = new ArrayList<>(mnemonics);
        >>>> that's why we get the npe here
                        Platform.runLater(() -> mnemonicsToRemove.forEach(scene::removeMnemonic));

        Note that this is a different root than the other NPEs (JDK-8244081, JDK-8244110) around MenuButtonSkin, it's uneffected by cleaning up in the sceneListener and/or in dispose. There seem to be several paths that remove the accelerators from the scene, probably need to find them all and guard against null (or funnel into a single cleanup, don't know if that's possible)


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                aghaisas Ajit Ghaisas
                fastegal Jeanette Winzenburg
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