当我按下模态按钮时，导航栏消失

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js"></script>
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet"/>

<nav class="navbar navbar-dark bg-dark nav-custom">
  <div class="container-fluid">
    <a class="navbar-brand p-3 m-0 h1" href="index.html">
      < Back</a>
  </div>


  <!-- Button trigger modal -->
  <button type="button" class="btn btn-primary" data-bs-toggle="modal" data-bs-target="#exampleModal">
                        Launch demo modal
                    </button>

  <!-- Modal -->
  <div class="modal fade" id="exampleModal" tabindex="-1" aria-labelledby="exampleModalLabel" aria-hidden="true">
    <div class="modal-dialog modal-fullscreen-lg-down">
      <div class="modal-content">
        <div class="modal-header">
          <h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
          <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
        </div>
        <div class="modal-body">
          ...
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
          <button type="button" class="btn btn-primary">Save changes</button>
        </div>
      </div>
    </div>
  </div>