fix: return empty Text component if renderer is updated with null (#8168)

Add the same behavior as when `createComponenet` is called and the
component renderer returns `null` to the `updateComponent` method. That
prevents a NPE in the `AbstractComponentDataGenerator#refreshData`
method when it tries to call `getElement` in the updated component.

Fixes #6825

Author: DiegoCardoso

vaadin-messages-flow-parent/vaadin-messages-flow/src/test/java/com/vaadin/flow/component/messages/tests/MessageListTest.java

@@ -40,17 +40,21 @@ public class MessageListTest {
     private MessageList messageList;
     private MessageListItem item1;
     private MessageListItem item2;
+    private UI ui;
 
     @Before
     public void setup() {
         messageList = new MessageList();
         item1 = new MessageListItem();
+        ui = new UI();
+        UI.setCurrent(ui);
         item2 = new MessageListItem();
     }
 
     @After
     public void tearDown() {
         UI.setCurrent(null);
+        ui = null;
     }
 
     @Test(expected = UnsupportedOperationException.class)
@@ -94,7 +98,6 @@ public void setItems_containsNullItem_throws() {
 
     @Test
     public void setImageAsStreamResource_overridesImageUrl() {
-        UI.setCurrent(new UI());
         item1.setUserImage("foo/bar");
         item1.setUserImageResource(new StreamResource("message-list-img",
                 () -> getClass().getResourceAsStream("baz/qux")));
@@ -105,7 +108,6 @@ public void setImageAsStreamResource_overridesImageUrl() {
 
     @Test
     public void setImageAsUrl_streamResourceBecomesNull() {
-        UI.setCurrent(new UI());
         item1.setUserImageResource(new StreamResource("message-list-img",
                 () -> getClass().getResourceAsStream("baz/qux")));
         item1.setUserImage("foo/bar");
@@ -114,7 +116,6 @@ public void setImageAsUrl_streamResourceBecomesNull() {
 
     @Test
     public void setImageHandler_overridesImageUrl() {
-        UI.setCurrent(new UI());
         item1.setUserImage("foo/bar");
         item1.setUserImageHandler(
                 DownloadHandler.fromInputStream(data -> new DownloadResponse(
@@ -127,7 +128,6 @@ public void setImageHandler_overridesImageUrl() {
 
     @Test
     public void setImageHandler_streamResourceBecomesNull() {
-        UI.setCurrent(new UI());
         item1.setUserImageHandler(
                 DownloadHandler.fromInputStream(data -> new DownloadResponse(
                         getClass().getResourceAsStream("baz/qux"),

vaadin-renderer-flow-parent/vaadin-renderer-flow/src/main/java/com/vaadin/flow/data/renderer/ComponentDataGenerator.java

@@ -101,16 +101,20 @@ public void generateData(T item, ObjectNode jsonObject) {
 
     @Override
     protected Component createComponent(T item) {
-        Component c = componentRenderer.createComponent(item);
-        if (c == null) {
-            c = new Text("");
-        }
-        return c;
+        return ensureNonNullComponent(componentRenderer.createComponent(item));
     }
 
     @Override
     protected Component updateComponent(Component currentComponent, T item) {
-        return componentRenderer.updateComponent(currentComponent, item);
+        return ensureNonNullComponent(
+                componentRenderer.updateComponent(currentComponent, item));
+    }
+
+    private Component ensureNonNullComponent(Component component) {
+        if (component == null) {
+            return new Text("");
+        }
+        return component;
     }
 
     @Override

vaadin-renderer-flow-parent/vaadin-renderer-flow/src/test/java/com/vaadin/flow/data/renderer/ComponentRendererTest.java

@@ -152,6 +152,11 @@ public void nullValues() {
         Assert.assertNotNull(
                 "Placeholder component should be generated for null values", c);
         Assert.assertEquals(0, c.getElement().getChildCount());
+
+        c = cdg.updateComponent(c, "foo");
+        Assert.assertNotNull(
+                "Placeholder component should be generated for null values", c);
+        Assert.assertEquals(0, c.getElement().getChildCount());
     }
 
     private void attachElement(UI ui, Element contentTemplate) {