Bootstrap starter template navbar toggler keep rolling up

I using bootstrap starter template and when i'm using it on mobile, the navbar toggler keep rolling up when you click on the button. Causing the user not able to navigate to other pages as the toggler rolls up immediately once the user click the button. Here's the link to my template:http://ift.tt/2A7V9qw Here's my code:

Pooptimer

    <div class="collapse navbar-collapse" id="  navbarsExampleDefault">

    <ul class="navbar-nav mr-auto">
      <li class="nav-item active">
        <a class="nav-link" href="/timers/new">Timer<span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="/users/profile">Profile</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="/about">About</a>
      </li>
      <li class="nav-item">
        <%if logged_in? %>

        <%= link_to 'Sign out', sign_out_path, method: :delete ,:class => "nav-link" %>
        <% else %>
        <%= logged_in? %>
        <a class="nav-link " href="/login">Sign In</a>
         <% end %>
      </li>

    </div>
</nav>