Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function "custom_single_template" not found or invalid function name in /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/wp-includes/class-wp-hook.php:326 Stack trace: #0 /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/wp-includes/plugin.php(205): WP_Hook->apply_filters('', Array) #1 /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/wp-includes/template.php(103): apply_filters('single_template', '', 'single', Array) #2 /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/wp-includes/template.php(571): get_query_template('single', Array) #3 /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/wp-includes/template-loader.php(81): get_single_template() #4 /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/wp-blog-header.php(19): require_once('/data03/virt124...') #5 /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/index.php(17): require('/data03/virt124...') #6 {main} thrown in /data03/virt124305/domeenid/www.kadrinatuulikud.ee/htdocs/wp-includes/class-wp-hook.php on line 326