当我尝试在 React 中返回数组的长度时遇到问题。错误是“TypeError：无法读取未定义的属性‘length’”。我已将其放在另一个子组件中并且它可以工作，但是当我在 NavBar 中尝试时它只显示错误。

import React from 'react';
import './NavBar.css';
import { FontAwesomeIcon } from '@fortawesome/react-fontawesome';
import { faCartPlus } from '@fortawesome/free-solid-svg-icons';

const NavBar = (props) => {
    const order = props.cart
    return (
        <div className='container-fluid'>
            <nav className="navbar navbar-expand-lg navbar-light  justify-content-md-between justify-content-center flex-wrap">
                <a href="/"><img src="https://i.ibb.co/NZcQbJM/logo2.png" alt="Red Onion Food"/></a>

                <div>
                    <a href="/" className='nav-item' > <FontAwesomeIcon icon={faCartPlus} /></a>
                    <a href="/" className='nav-item login'>Login</a>
                    <a href="/" className='nav-item'><button className="btn btn-danger btn-rounded">Sign Up</button></a>
                </div>
            </nav>
            <div><h6>{order.length}</h6></div>
        </div>
    );
};

export default NavBar;

I dont know what to do!!!