Clear errors when clearing binder fields (#9364)

Calling readBean(null) should reset the form state to the initial state
it was in before calling writeBean(bean)

server/src/main/java/com/vaadin/data/Binder.java

@@ -1561,7 +1561,10 @@ public class Binder<BEAN> implements Serializable {
      * Clear all the bound fields for this binder.
      */
     private void clearFields() {
-        bindings.forEach(binding -> binding.getField().clear());
+        bindings.forEach(binding -> {
+            binding.getField().clear();
+            clearError(binding.getField());
+        });
         if (hasChanges()) {
             fireStatusChangeEvent(false);
         }

server/src/test/java/com/vaadin/data/BinderTest.java

@@ -441,6 +441,26 @@ public class BinderTest extends BinderTestBase<Binder<Person>, Person> {
         assertTrue(textField.isRequiredIndicatorVisible());
     }
 
+    @Test
+    public void readNullBeanRemovesError() {
+        TextField textField = new TextField();
+        binder.forField(textField).asRequired("foobar")
+                .bind(Person::getFirstName, Person::setFirstName);
+        Assert.assertTrue(textField.isRequiredIndicatorVisible());
+        Assert.assertNull(textField.getErrorMessage());
+
+        binder.readBean(item);
+        Assert.assertNull(textField.getErrorMessage());
+
+        textField.setValue(textField.getEmptyValue());
+        Assert.assertTrue(textField.isRequiredIndicatorVisible());
+        Assert.assertNotNull(textField.getErrorMessage());
+
+        binder.readBean(null);
+        assertTrue(textField.isRequiredIndicatorVisible());
+        Assert.assertNull(textField.getErrorMessage());
+    }
+
     @Test
     public void setRequired_withErrorMessageProvider_fieldGetsRequiredIndicatorAndValidator() {
         TextField textField = new TextField();