🎬 fix Form demo warning (#21576)

close #21543

components/form/demo/form-context.md

@@ -14,7 +14,7 @@ title:
 Use `Form.Provider` to process data between forms. In this case, submit button is in the Modal which is out of Form. You can use `form.submit` to submit form. Besides, we recommend native `<Button htmlType="submit" />` to submit a form.
 
 ```tsx
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useRef } from 'react';
 import { Form, Input, InputNumber, Modal, Button, Avatar, Typography } from 'antd';
 import { SmileOutlined, UserOutlined } from '@ant-design/icons';
 
@@ -34,8 +34,17 @@ interface ModalFormProps {
 const ModalForm: React.FC<ModalFormProps> = ({ visible, onCancel }) => {
   const [form] = Form.useForm();
 
+  const prevVisibleRef = useRef();
   useEffect(() => {
-    form.resetFields();
+    prevVisibleRef.current = visible;
+  }, [visible]);
+  const prevVisible = prevVisibleRef.current;
+
+  useEffect(() => {
+    console.log(visible, prevVisible);
+    if (!visible && prevVisible) {
+      form.resetFields();
+    }
   }, [visible]);
 
   const onOk = () => {