Call to a member function using_index_permalinks() on null

[nigrosimone]

From official forum https://wordpress.org/support/topic/fatal-error-util_pageurls-php/

Fatal error: Call to a member function using_index_permalinks() on null in /data/www/14157/kosmonautix_cz/wp-content/plugins/w3-total-cache/Util_PageUrls.php on line 503

w3tc doesn't check if global $wp_rewrite;is initialized:

if ( $wp_rewrite->using_index_permalinks() && ( $pagenum > 1 || '' != $request ) )
    $base .= 'index.php/';

My fix is a simple check

if ( !is_null($wp_rewrite) && $wp_rewrite->using_index_permalinks() && ( $pagenum > 1 || '' != $request ) )
    $base .= 'index.php/';