<nav class="navbar" role="navigation">
         <div class="navbar-header">
              <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-ex1-collapse">

<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>

<a class="navbar-brand" href="<?php bloginfo('url'); ?>">
<?php bloginfo('name'); ?>
</a>
</div>

<!-- Collect the nav links, forms, and other content for toggling -->

<?php
$backup_query = $wp_query;
$wp_query = new WP_Query(array('post_type' => 'post'));
wp_nav_menu( array(
'menu'       => 'header-menu',
'theme_location' => 'menu-principale',
'depth'      => 2,
'container'  => 'div',
'container_class'   => 'collapse navbar-collapse navbar-ex1-collapse',
'menu_class' => 'nav navbar-nav smooth',
'fallback_cb' => 'wp_bootstrap_navwalker::fallback',
'walker' => new wp_bootstrap_navwalker())
);

$wp_query = $backup_query;
?>
</nav><!-- /Nav -->