Toggler menu open then disappears?

[bnmz41998]

Hello, I've inserted a toggler into my navbar but the problem I'm having is the toggler opens you click it, drops down but soon as it drops down it disappears. Then when you click the menu again you can see it closing, so it seems when it's in a fully open state it isn't visible.

My code is herE:

<nav class="navbar navbar-expand-md bg-danger navbar-light">
  <button
    class="navbar-toggler"
    type="button"
    data-toggle="collapse"
    data-target="#navbarTogglerDemo01"
    aria-controls="navbarTogglerDemo01"
    aria-expanded="false"
    aria-label="Toggle navigation"
  >
    <span class="navbar-toggler-icon"></span>
  </button>

  <a class="navbar-brand">ELectro</a>
  <div class="collapse navbar-collapse" id="navbarTogglerDemo01">
    <ul class="navbar-nav ms-auto">
      <li class="nav-item">
        <a class="nav-link" href="">About</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="">Portfolio</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="">Portfolio</a>
      </li>
    </ul>
  </div>
</nav>