fix: fail when adding null component event listener (#17276)

Currently, it is possible to add component event listeners pointing to a null
reference. At runtime an NPE is thrown, but for an internal wrapper class,
making difficult to spot where the null reference comes from.
This change will throw immediately if null is given as a component event
listener.

Author: mcollovati

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

@@ -141,6 +141,11 @@ private <T extends ComponentEvent<?>> Registration addListenerInternal(
             Class<T> eventType, ComponentEventListener<T> listener,
             Consumer<DomListenerRegistration> domListenerConsumer) {
 
+        if (listener == null) {
+            throw new IllegalArgumentException(
+                    "component event listener cannot be null");
+        }
+
         ListenerWrapper<T> wrapper = new ListenerWrapper<>(listener);
 
         boolean isDomEvent = addDomTriggerIfNeeded(eventType, wrapper);

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

@@ -21,7 +21,9 @@
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.junit.Assert;
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.ExpectedException;
 import org.mockito.MockedStatic;
 import org.mockito.Mockito;
 
@@ -41,6 +43,9 @@
 
 public class ComponentEventBusTest {
 
+    @Rule
+    public ExpectedException exceptionRule = ExpectedException.none();
+
     private static class EventTracker<T extends ComponentEvent<?>>
             implements ComponentEventListener<T> {
         private AtomicInteger eventHandlerCalled = new AtomicInteger(0);
@@ -740,4 +745,13 @@ public void addListener_eventDataExpressionsPresent_constantPoolKeyNotCreatedAft
             util.verifyNoInteractions();
         }
     }
+
+    @Test
+    public void addListener_nullListener_failFast() {
+        exceptionRule.expect(IllegalArgumentException.class);
+        exceptionRule.expectMessage("component event listener cannot be null");
+
+        final TestButton button = new TestButton();
+        button.addListener(ServerEvent.class, null);
+    }
 }

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

@@ -585,10 +585,10 @@ public void reattachComponent_detachListenerIsAddedOnEveryAttach_listenOnUIIsClo
 
         Registration registration = Mockito.mock(Registration.class);
         AtomicInteger count = new AtomicInteger();
-        Mockito.when(ui.addDetachListener(any())).thenAnswer(invocation -> {
+        Mockito.doAnswer(invocation -> {
             count.incrementAndGet();
             return registration;
-        });
+        }).when(ui).addDetachListener(any());
 
         Component[] components = new Component[] { ui };