fix: setState(name, null) should send JSON null to client

flow-server/src/main/java/com/vaadin/flow/internal/JacksonUtils.java

@@ -519,6 +519,9 @@ public static <T> T readValue(JsonNode jsonValue,
      * @return converted JSON value
      */
     public static BaseJsonNode writeValue(Object object) {
+        if (object == null) {
+            return (BaseJsonNode) objectMapper.nullNode();
+        }
         return objectMapper.valueToTree(object);
     }
 

flow-server/src/test/java/com/vaadin/flow/internal/JacksonUtilsTest.java

@@ -36,6 +36,7 @@
 import tools.jackson.databind.JsonNode;
 import tools.jackson.databind.ObjectMapper;
 import tools.jackson.databind.node.ArrayNode;
+import tools.jackson.databind.node.BaseJsonNode;
 import tools.jackson.databind.node.DoubleNode;
 import tools.jackson.databind.node.ObjectNode;
 
@@ -490,4 +491,11 @@ public void toFileJson() throws JacksonException {
 
     }
 
+    @Test
+    public void writeValue_nullReturnsNullNode() {
+        BaseJsonNode result = JacksonUtils.writeValue(null);
+        Assert.assertTrue("Expected NullNode", result.isNull());
+        Assert.assertEquals(mapper.nullNode(), result);
+    }
+
 }

flow-tests/test-react-adapter/src/main/java/com/vaadin/flow/ReactAdapterView.java

@@ -35,6 +35,10 @@ public ReactAdapterView() {
                 (event) -> input.setValue("set value"));
         setValueButton.setId("setValueButton");
 
+        var setNullButton = new NativeButton("Set null",
+                (event) -> input.setValue(null));
+        setNullButton.setId("setNullValueButton");
+
         var getOutput = new Span();
         getOutput.setId("getOutput");
 
@@ -43,7 +47,7 @@ public ReactAdapterView() {
         getValueButton.setId("getValueButton");
 
         add(new Div(input, listenerOutput), new Div(setValueButton),
-                new Div(getValueButton, getOutput));
+                new Div(setNullButton), new Div(getValueButton, getOutput));
     }
 
 }

flow-tests/test-react-adapter/src/test/java/com/vaadin/flow/ReactAdapterIT.java

@@ -81,6 +81,19 @@ public void validateListener() {
                 $(SpanElement.class).id("listenerOutput").getText());
     }
 
+    @Test
+    public void validateSetNullState() {
+        open();
+
+        waitForDevServer();
+
+        $(NativeButtonElement.class).id("setNullValueButton").click();
+
+        // getPropertyString returns null for null/undefined values
+        String value = getAdapterElement().getPropertyString("value");
+        Assert.assertNull("Expected null value, not string 'null'", value);
+    }
+
     private TestBenchElement getAdapterElement() {
         return $("react-input").first();
     }