PHP Notice:  Undefined offset: 0 in extensions/Html2Wiki/specials/SpecialHtml2Wiki.php on line 336

extension.json: "callback": "SpecialHtml2Wiki::checkEnvironment",

334     static function checkEnvironment() {
335         global $wgNamespacesWithSubpages;
336             if ( $wgNamespacesWithSubpages[NS_MAIN] !== true ) {
337                 die( "This extension requires \$wgNamespacesWithSubpages set to TRUE in the MAIN namespace.
338                 Please add \n
339                 \$wgNamespacesWithSubpages[NS_MAIN] = true;\n to your LocalSettings.php" );
340             }