All dropdown to be links when using the bootstrap nav walked or under strap theme.

understrap-child-allow-dropdown-links.php

<?php

/**
 * Allow 
 *
 * @param [type] $atts
 * @param [type] $item
 * @param [type] $args
 * @param [type] $depth
 * @return void
 */
function bc_set_link_href_for_dropdowns( $atts, $item, $args, $depth ) {
	if ( isset( $atts['class'] ) && 'dropdown-toggle nav-link' == $atts['class'] ) {
		$atts['href'] = ! empty( $item->url ) ? $item->url : '#';
	}
	return $atts;
}
add_filter( 'nav_menu_link_attributes', 'bc_set_link_href_for_dropdowns', 10, 4 );