fix: Prevent NPE in ShortcutRegistration when lifecycleOwner is null (#23465) (#23478)

Add null guard in fireShortcutEvent() to handle the case where a
KeyDown event fires after the shortcut's lifecycle owner has been
detached and remove() has set lifecycleOwner to null.

Fixes #19769

Co-authored-by: Artur Signell <artur@vaadin.com>

Author: vaadin-bot

flow-server/src/main/java/com/vaadin/flow/component/ShortcutRegistration.java

@@ -673,6 +673,9 @@ private Component getComponentEventSource(int listenOnIndex) {
     }
 
     private void fireShortcutEvent(Component component) {
+        if (lifecycleOwner == null) {
+            return;
+        }
         if (ancestorsOrSelfAreVisible(lifecycleOwner) && (lifecycleOwner
                 .getElement().isEnabled()
                 || DisabledUpdateMode.ALWAYS.equals(getDisabledUpdateMode()))) {

flow-server/src/test/java/com/vaadin/flow/component/ShortcutRegistrationTest.java

@@ -16,6 +16,7 @@
 
 package com.vaadin.flow.component;
 
+import java.lang.reflect.Field;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -617,6 +618,32 @@ public void constructedRegistration_lifecycleOnwerHasInvisibleParent_shorcutEven
         Assert.assertNull(event.get());
     }
 
+    @Test
+    public void constructedRegistration_lifecycleOwnerNulledBeforeKeyEvent_noNPE()
+            throws Exception {
+        AtomicReference<ShortcutEvent> event = new AtomicReference<>();
+
+        ShortcutRegistration registration = new ShortcutRegistration(
+                lifecycleOwner, () -> listenOn, event::set, Key.KEY_A);
+
+        mockLifecycle(true);
+        clientResponse();
+
+        // Simulate the race condition: null out lifecycleOwner while
+        // the KeyDown listener is still registered
+        Field field = ShortcutRegistration.class
+                .getDeclaredField("lifecycleOwner");
+        field.setAccessible(true);
+        field.set(registration, null);
+
+        // Fire KeyDown event — should not throw NPE
+        listenOn[0].getEventBus()
+                .fireEvent(new KeyDownEvent(listenOn[0], Key.KEY_A.toString()));
+
+        // Shortcut event should not have been fired
+        Assert.assertNull(event.get());
+    }
+
     @Test
     public void constructedRegistration_lifeCycleOwnerIsDetached_detachListenerIsDeregisteredFromListenOnComponents() {
         AtomicReference<ComponentEventListener> detachListener = new AtomicReference<>();