add_filter( 'nav_menu_link_attributes', 'prefix_add_active_class_to_anchor', 10, 3 );
function prefix_add_active_class_to_anchor( $atts, $item, $args ) {
	if ( false === prefix_is_active_wp_bootstrap_navwalker_item( $item, $args ) ) {
		return $atts;
	}

	if ( isset( $atts['class'] ) ) {
		$atts['class'] .= ' active';
	} else {
		$atts['class'] = 'active';
	}
	return $atts;
}

function prefix_is_active_wp_bootstrap_navwalker_item( $item, $args ) {
	if ( ! property_exists( $args, 'walker' ) || ! is_a( $args->walker, 'WP_Bootstrap_Navwalker') ) {
		return false;
 	}
	if ( ! $item->current && ! $item->current_item_ancestor ) {
		return false;
 	}

	return true;
}

add_filter( 'nav_menu_css_class', 'prefix_remove_active_class_from_li', 10, 3 );
function prefix_remove_active_class_from_li( $classes, $item, $args ) {
	if ( false === prefix_is_active_wp_bootstrap_navwalker_item( $item, $args ) ) {
		return $classes;
	}

	return array_diff( $classes, array( 'active' ) ) ;
}