PHP Notice on undefined index: add_bu_links.

[ravigehlot]

I confirm that this notice comes up for PHP 7.1, 7.2 and 7.3:

[15-Sep-2019 08:22:32 America/New_York] PHP Notice: Undefined index: add_bu_links in /wp-content/plugins/bu-navigation/extras/bu-navigation-external-links.php on line 128

This is the block of code in question:

function bu_navigation_add_links_to_pages_list( $pages, $r ){
	if( $r['add_bu_links'] ){

		$post_parents = array();
		foreach($pages as $page){
			if( array_search( $page->post_parent, $post_parents ) === false ){
				$post_parents[] = $page->post_parent;
			}
		}

		
		$args = array(
			'post_type' => BU_NAVIGATION_LINK_POST_TYPE,
			'post_parent__in' => $post_parents,  
			'posts_per_page'   => -1,
		);

		$bu_query = new WP_Query( $args );
		$bu_links = $bu_query->posts;

		for($i = 0; $i < count($bu_links); $i++ ){
			$bu_links[$i]->post_type = 'page';
		}
		$pages = array_merge( $pages, $bu_links );
		usort( $pages, function ( $a, $b ){
			if(  $a->post_parent == $b->post_parent ){
				return $a->menu_order > $b->menu_order ? 1 : -1;
			}
			return $a->post_parent > $b->post_parent ? 1 : -1;
		});
	}
	return $pages;
}