Error previewing theme in admin

Hello.

I have downloaded this theme through my WP admin but when I try to preview it, it gives me PHP error about capabilities.php.

Here is the message :

Fatal error: Uncaught Error: Call to undefined function wp_get_current_user() in /home/XXXXX/wp-includes/capabilities.php:877 Stack trace: #0 /home/XXXXX/wp-includes/theme-previews.php(17): current_user_can('switch_themes') #1 /home/XXXXX/wp-includes/class-wp-hook.php(310): wp_get_theme_preview_path('twentytwenty') #2 /home/XXXXX/wp-includes/plugin.php(205): WP_Hook->apply_filters('twentytwenty', Array) #3 /home/XXXXX/wp-includes/theme.php(317): apply_filters('template', 'twentytwenty') #4 /home/XXXXX/wp-includes/theme.php(328): get_template() #5 /home/XXXXX/wp-includes/l10n.php(1306): get_template_directory() #6 /home/XXXXX/wp-includes/l10n.php(1331): _load_textdomain_just_in_time('simple-lightbox') #7 /home/XXXXX/wp-includes/l10n.php(185): get_translations_for_domain('simple-lightbox') #8 /home/XXXXX/wp-includes/l10n.php(297): translate('Simple Lightbox', 'simple-lightbox') #9 /home/XXXXX/wp-content/plugins/simple-lightbox/main.php(27): __('Simple Lightbox', 'simple-lightbox') #10 /home/XXXXX/wp-settings.php(462): include_once('/XXXXX...') #11 /home/XXXXX/wp-config.php(98): require_once('/XXXXX...') #12 /home/XXXXX/wp-load.php(50): require_once('/XXXXX...') #13 /home/XXXXX/wp-admin/admin.php(34): require_once('/XXXXX...') #14 /home/XXXXX/wp-admin/site-editor.php(12): require_once('/XXXXX...') #15 {main} thrown in /home/XXXXX/wp-includes/capabilities.php on line 877

Any idea of the problem ?

Thanks.