antd在componentDidMount內(nèi)部聲明的<FormItem />生成的輸入欄失效

如果我在render外部聲明了<FormItem />（如在componentDidMount內(nèi)），然后再渲染它，生成的表單是不能被輸入的，如：

import React from "react";
import ReactDOM from "react-dom";
import "antd/dist/antd.css";
import "./index.css";
import { Form, Icon, Input, Button, Checkbox } from "antd";
const FormItem = Form.Item;

class CustomizedForm extends React.Component {
  handleSubmit = e => {
    e.preventDefault();
  };
  state = {
    ready: false
  };
  componentDidMount() {
    const { getFieldDecorator } = this.props.form;
    this.formItem = (
      <FormItem>{getFieldDecorator("field1")(<Input />)}</FormItem>
    );
    this.setState({ ready: true });
  }
  render() {
    return (
      <Form onSubmit={this.handleSubmit} className="login-form">
        {this.state.ready === true ? this.formItem : null}
      </Form>
    );
  }
}

const WrappedNormalLoginForm = Form.create()(CustomizedForm);

ReactDOM.render(
  <WrappedNormalLoginForm />,
  document.getElementById("container")
);

CodeSandBox

但是在render內(nèi)部聲明則可以正常使用，如：

import React from "react";
import ReactDOM from "react-dom";
import "antd/dist/antd.css";
import "./index.css";
import { Form, Icon, Input, Button, Checkbox } from "antd";
const FormItem = Form.Item;

class CustomizedForm extends React.Component {
  handleSubmit = e => {
    e.preventDefault();
  };
  render() {
    const { getFieldDecorator } = this.props.form;
    this.formItem = (
      <FormItem>{getFieldDecorator("field1")(<Input />)}</FormItem>
    );
    return (
      <Form onSubmit={this.handleSubmit} className="login-form">
        {this.formItem}
      </Form>
    );
  }
}

const WrappedNormalLoginForm = Form.create()(CustomizedForm);

ReactDOM.render(
  <WrappedNormalLoginForm />,
  document.getElementById("container")
);

CodeSandBox

請(qǐng)問(wèn)會(huì)造成這個(gè)問(wèn)題的原因是什么？
因?yàn)榭雌饋?lái)兩種方式幾乎是一樣的，以及是否能在render外部生成<FormItem />并且正常使用它？
謝謝 ！