fix: ensure validator passed in Form rerenders & prevent null schema errors

This commit fixes two potential issues:

1. Updates `createComponent` test utility to ensure Form components always receive
   a validator prop when rerendering, preventing potential test failures

2. Adds null checks for rootSchema in SchemaUtils to prevent errors when
   schema is undefined or null

Author: chathuraa

packages/core/test/test_utils.js

@@ -16,8 +16,11 @@ export function createComponent(Component, props) {
   const comp = <Component onSubmit={onSubmit} onError={onError} onChange={onChange} {...props} />;
   const { container, rerender } = render(comp);
 
-  const rerenderFunction = (props) =>
-    rerender(<Component onSubmit={onSubmit} onError={onError} onChange={onChange} {...props} />);
+  const rerenderFunction = (newProps) => {
+    // For Form components, ensure validator is always passed
+    const propsWithValidator = Component === Form && !newProps.validator ? { validator, ...newProps } : newProps;
+    return rerender(<Component onSubmit={onSubmit} onError={onError} onChange={onChange} {...propsWithValidator} />);
+  };
   const node = findDOMNode(container).firstElementChild;
 
   return { comp, node, onChange, onError, onSubmit, rerender: rerenderFunction };

packages/utils/src/createSchemaUtils.ts

@@ -60,7 +60,7 @@ class SchemaUtils<T = any, S extends StrictRJSFSchema = RJSFSchema, F extends Fo
     experimental_defaultFormStateBehavior: Experimental_DefaultFormStateBehavior,
     experimental_customMergeAllOf?: Experimental_CustomMergeAllOf<S>,
   ) {
-    if (rootSchema[SCHEMA_KEY] === JSON_SCHEMA_DRAFT_2020_12) {
+    if (rootSchema && rootSchema[SCHEMA_KEY] === JSON_SCHEMA_DRAFT_2020_12) {
       this.rootSchema = makeAllReferencesAbsolute(rootSchema, get(rootSchema, ID_KEY, '#'));
     } else {
       this.rootSchema = rootSchema;
@@ -102,9 +102,12 @@ class SchemaUtils<T = any, S extends StrictRJSFSchema = RJSFSchema, F extends Fo
     experimental_defaultFormStateBehavior = {},
     experimental_customMergeAllOf?: Experimental_CustomMergeAllOf<S>,
   ): boolean {
+    // If either validator or rootSchema are falsy, return false to prevent the creation
+    // of a new SchemaUtilsType with incomplete properties.
     if (!validator || !rootSchema) {
       return false;
     }
+
     return (
       this.validator !== validator ||
       !deepEquals(this.rootSchema, rootSchema) ||