PHP Fatal error: Uncaught Error: Call to undefined function wp_get_current_user(

I’m getting the following error when enabling this plugin:

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to undefined function wp_get_current_user() in /var/www/vhosts/domain.com/httpdocs/wp-includes/capabilities.php:590\nStack trace:\n#0 /var/www/vhosts/domain.com/httpdocs/wp-content/plugins/hwp-utility-plugin/hwp-utility.php(121): current_user_can('administrator')\n#1 /var/www/vhosts/domain.com/httpdocs/wp-includes/class-wp-hook.php(286): hwp_pre_user_query(Object(WP_User_Query))\n#2 /var/www/vhosts/domain.com/httpdocs/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array)\n#3 /var/www/vhosts/domain.com/httpdocs/wp-includes/plugin.php(515): WP_Hook->do_action(Array)\n#4 /var/www/vhosts/domain.com/httpdocs/wp-includes/class-wp-user-query.php(576): do_action_ref_array('pre_user_query', Array)\n#5 /var/www/vhosts/domain.com/httpdocs/wp-includes/class-wp-user-query.php(77): WP_User_Query->prepare_query(Array)\n#6 /var/www/vhosts/domain.com/httpdocs/wp-includes/user.php(561): WP_User_Query->__construct(Arr...\n'

Here is the relevant plugin code that uses ‘current_user_can’ – causes a white screen when WPFC is enabled and user is logged out:

/**
  * Hide Administrator from User list
  */
add_action( 'pre_user_query', 'hwp_pre_user_query' );
function hwp_pre_user_query( $user_search ) {
    if ( !current_user_can( 'administrator' ) ) { // Is Not Administrator - Remove Administrator
        global $wpdb;

        $user_search->query_where = str_replace(
            'WHERE 1=1', 
            "WHERE 1=1 AND {$wpdb->users}.ID IN (
              SELECT {$wpdb->usermeta}.user_id FROM $wpdb->usermeta 
              WHERE {$wpdb->usermeta}.meta_key = '{$wpdb->prefix}capabilities'
              AND {$wpdb->usermeta}.meta_value NOT LIKE '%administrator%' )", 
            $user_search->query_where
        );
    }
}