Fix 4821 by no saving undefined values into an object for a null type field (#4839)

* Fix 4821 by no saving undefined values into an object for a null type field
Fixed #4821 by not saving an undefined field value into an object when that field has the `null` type
- Updated `getDefaultFormState()` to not save an undefined field value into an object when the type is `null` and `excludeObjectChildren` is provided
  - Updated the tests to verify this new fix
- Updated the `CHANGELOG.md` accordingly

# Conflicts:
#	CHANGELOG.md

* - Responded to reviewer feedback

Author: heath-freenome

CHANGELOG.md

@@ -60,6 +60,10 @@ should change the heading of the (upcoming) version to include a major version b
 
 - Updated `FieldTemplate` to skip label and description rendering for checkbox widgets, fixing ([#4742](https://github.com/rjsf-team/react-jsonschema-form/issues/4742))
 
+## @rjsf/utils
+
+- Updated `getDefaultFormState()` to not save an undefined field value into an object when the type is `null` and `excludeObjectChildren` is provided, fixing [#4821](https://github.com/rjsf-team/react-jsonschema-form/issues/4821)
+
 ## Dev / docs / playground
 
 - Updated the `OptionsDrawer` of the playground to add `idPrefix` and `idSeparator` fields

packages/utils/src/schema/getDefaultFormState.ts

@@ -121,6 +121,7 @@ export function computeDefaultBasedOnSchemaTypeAndDefaults<T = any, S extends St
  * @param experimental_defaultFormStateBehavior - Optional configuration object, if provided, allows users to override
  *        default form state behavior
  * @param isConst - Optional flag, if true, indicates that the schema has a const property defined, thus we should always return the computedDefault since it's coming from the const.
+ * @param isNullType - The type of the schema is null
  */
 function maybeAddDefaultToObject<T = any>(
   obj: GenericObjectType,
@@ -131,6 +132,7 @@ function maybeAddDefaultToObject<T = any>(
   requiredFields: string[] = [],
   experimental_defaultFormStateBehavior: Experimental_DefaultFormStateBehavior = {},
   isConst = false,
+  isNullType = false,
 ) {
   const { emptyObjectFields = 'populateAllDefaults' } = experimental_defaultFormStateBehavior;
 
@@ -141,7 +143,7 @@ function maybeAddDefaultToObject<T = any>(
   } else if (includeUndefinedValues === 'excludeObjectChildren') {
     // Fix for Issue #4709: When in 'excludeObjectChildren' mode, don't set primitive fields to empty objects
     // Only add the computed default if it's not an empty object placeholder for a primitive field
-    if (!isObject(computedDefault) || !isEmpty(computedDefault)) {
+    if ((isNullType && computedDefault !== undefined) || !isObject(computedDefault) || !isEmpty(computedDefault)) {
       obj[key] = computedDefault;
     }
     // If computedDefault is an empty object {}, don't add it - let the field stay undefined
@@ -517,6 +519,7 @@ export function getObjectDefaults<T = any, S extends StrictRJSFSchema = RJSFSche
           retrievedSchema.required,
           experimental_defaultFormStateBehavior,
           hasConst,
+          propertySchema?.type === 'null',
         );
 
         return acc;

packages/utils/test/schema/getDefaultFormStateTest.ts

@@ -376,6 +376,53 @@ export default function getDefaultFormStateTest(testValidator: TestValidatorType
         });
       });
 
+      describe("an object with an null-type property and includeUndefinedValues is 'excludeObjectChildren'", () => {
+        const schema: RJSFSchema = {
+          type: 'object',
+          properties: {
+            optionalNullProperty: {
+              type: 'null',
+            },
+            requiredProperty: {
+              type: 'string',
+              default: 'foo',
+            },
+          },
+          required: ['requiredProperty'],
+        };
+        const includeUndefinedValues = 'excludeObjectChildren';
+        const expected = {
+          requiredProperty: 'foo',
+        };
+
+        test('getDefaultFormState', () => {
+          expect(getDefaultFormState(testValidator, schema, undefined, schema, includeUndefinedValues)).toEqual(
+            expected,
+          );
+        });
+
+        test('computeDefaults', () => {
+          expect(
+            computeDefaults(testValidator, schema, {
+              rootSchema: schema,
+              includeUndefinedValues,
+            }),
+          ).toEqual(expected);
+        });
+
+        test('getDefaultBasedOnSchemaType', () => {
+          expect(
+            getDefaultBasedOnSchemaType(testValidator, schema, { rootSchema: schema, includeUndefinedValues }),
+          ).toEqual(expected);
+        });
+
+        test('getObjectDefaults', () => {
+          expect(getObjectDefaults(testValidator, schema, { rootSchema: schema, includeUndefinedValues })).toEqual(
+            expected,
+          );
+        });
+      });
+
       describe('an object with an additionalProperties', () => {
         const schema: RJSFSchema = {
           type: 'object',