dropdown toggle issue in navbar with ng-bootstrap (4) angular 2 (cli)

I got it working like this:

<nav class="navbar navbar-light bg-faded">
  <a class="navbar-brand" href="#">Navbar</a>
  <ul class="nav navbar-nav">
    <li class="nav-item active">
      <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
    </li>
    <li class="nav-item">
      <a class="nav-link" href="#">Link</a>
    </li>
    <li class="nav-item">
      <a class="nav-link" href="#">Link</a>
    </li>
    <li class="nav-item dropdown">
      <div ngbDropdown class="d-inline-block">
        <button class="btn btn-outline-primary" id="dropdownMenu1" ngbDropdownToggle>Toggle dropdown</button>
        <div ngbDropdownMenu class="dropdown-menu" aria-labelledby="dropdownMenu1">
          <button class="dropdown-item">Action - 1</button>
          <button class="dropdown-item">Another Action</button>
          <button class="dropdown-item">Something else is here</button>
        </div>
      </div>
    </li>
  </ul>
  <form class="form-inline float-xs-right">
    <input class="form-control" type="text" placeholder="Search">
    <button class="btn btn-outline-success" type="submit">Search</button>
  </form>
</nav>

As pointed above in the comment (link)

I ended up doing something like this:

In the html template i have this code:

 <nav id="mainNavbar" class="navbar navbar-expand-lg navbar-dark bg-dark fixed-top">
        <a class="navbar-brand" href="#">Navbar</a>
        <button class="navbar-toggler" type="button" data-toggle="collapse"
           data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent"
           aria-expanded="false" aria-label="Toggle navigation
           (click)="toggleCollapsed()">
          <span class="navbar-toggler-icon"></span>
        </button>
        <div id="navbarSupportedContent" [ngClass]="{'collapse': collapsed, 'navbar-collapse': true}">
......
</nav>

In the component i have this

 export class NavComponent {
      collapsed = true;
      toggleCollapsed(): void {
        this.collapsed = !this.collapsed;
      }
 }

modify your drown down html parts to this:

<li class="nav-item dropdown">
  <div ngbDropdown class="d-inline-block">
    <button class="btn btn-outline-primary" id="dropdownMenu1" 
      **ngbDropdownToggle**>use Attrib ngbDropdownToggle</button>

    <!-- use  ngbDropdownMenu -->
    <div **ngbDropdownMenu** class="dropdown-menu" aria-labelledby="dropdownMenu1">
      <button class="dropdown-item">Action - 1</button>
      <button class="dropdown-item">Another Action</button>
      <button class="dropdown-item">Something else is here</button>
    </div>
  </div>
</li>