Codeigniter 3.1 - Bootstrap 4导航栏不会崩溃

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Logo</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="navbarNav">
    <ul class="navbar-nav">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">About</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Services</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Contact</a>
      </li>
    </ul>
  </div>
</nav>

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Home extends CI_Controller {

    public function index()
    {
        $this->load->view('navbar');
        $this->load->view('home');
    }
}

$route['default_controller'] = 'home';
$route['404_override'] = '';
$route['translate_uri_dashes'] = FALSE;