array_key_exists exception on 1.7-rc10 with Gantry latest

[thekenshow]

Installed Sitemap plugin, enabled, browsed to '/sitemap', got the array_key_exists(): The first argument should be either a string or an integer error on line 78 of sitemap.php:

$lang_available = (empty($page_languages) || array_key_exists($current_lang, $page_languages));

[mahagr]

FYI: Not related to Grav 1.7 but PHP 7.4.

[thekenshow]

This is happening on PHP 7.2.24.

[mahagr]

Thanks.

[mahagr]

It looks like $language->getLanguage() and $language->getDefault() can both return null.

CC @rhukster

[mahagr]

@thekenshow Workaround is to set (default) language in system.yaml.