Fatal error: Uncaught Error: Call to undefined function is_main_site() in /home/KatarzynaCzajka/public_html/katarzynaczajka/wordpress/wp-includes/update.php:959 Stack trace: #0 {main} thrown in /home/KatarzynaCzajka/public_html/katarzynaczajka/wordpress/wp-includes/update.php on line 959