错误：元素类型无效，请检查渲染方法

Question

我正在学习React，整理我的导航栏时遇到了一些麻烦。

我有此错误：

public void handle(ActionEvent event) {
    PauseTransition delay = new PauseTransition(Duration.minutes(30));
    delay.setOnFinished(evt -> {
        // whatever you need to do in 30 minutes....
    });
    delay.play();
}

检查Error: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in, or you might have mixed up default and named imports. 的呈现方法。

在线查看表明，大多数情况下，这是命名约定的问题。但对于我的一生，我看不到任何错误。

这是我的文件：

导航

navigation

布局

import './Navigation.css';
import { Col, Navbar, Nav, NavItem } from 'react-bootstrap';
import { NavLink } from 'react-router-dom';
import { LinkContainer } from 'react-router-bootstrap';

const navigation = (props) => {
    return (
        <Col md={12} >
            <Navbar inverse collapseOnSelect>
                <Navbar.Header>
                    <Navbar.Brand>
                        <NavLink to={'/'} exact >Account-Owner</NavLink>
                    </Navbar.Brand>
                    <Navbar.Toggle />
                </Navbar.Header>
                <Navbar.Collapse>
                    <Nav>
                        <LinkContainer to={'/owner-list'} exact>
                            <NavItem eventKey={1}>
                                Owner Actions
                            </NavItem>
                        </LinkContainer>
                        <LinkContainer to={'/account-list'}>
                            <NavItem eventKey={2}>
                                Account Actions
                            </NavItem>
                        </LinkContainer>
                    </Nav>
                </Navbar.Collapse>
            </Navbar>
        </Col>
    )
}

export default navigation;

app.js

import { Container as Container, Row } from 'react-bootstrap';
import Navigation from '../Navigation/Navigation';

const layout = (props) => {
    return (
        <Container>
            <Row>
                <Navigation/>
            </Row>
            <main>
                {props.children}
            </main>
        </Container>
    )
}

export default layout;