[Pro Sites] - Disable Free level access. Only admin can access Free sites that have no pro level

ps-disable-free-front.php

<?php
/**
* Plugin Name: [Pro Sites] - Disable Free level access[Pro Sites] - Disable Free level access
* Plugin URI: https://premium.wpmudev.org/
* Description: Only admin can access Free sites that have no pro level
* Author: Panos Lyrakis @ WPMUDEV
* Author URI: https://premium.wpmudev.org/
* License: GPLv2 or later
*/

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

if ( ! class_exists( 'WPMUDEV_PS_Disable_Free_Front' ) ) {

    class WPMUDEV_PS_Disable_Free_Front {

        private static $_instance 		= null;
        private $_page_to_redirect 		= 41;
        
		public static function get_instance() {

			if ( is_null( self::$_instance ) ) {
				self::$_instance = new WPMUDEV_PS_Disable_Free_Front();
			}

			return self::$_instance;
		}

		private function __construct() {

			add_action( 'template_redirect', array( $this, 'maybe_disable_access' ), 20 );

		}

		public function maybe_disable_access() {

			global $psts, $blog_id;
			$main_site = get_network()->site_id;

			if ( is_admin() || current_user_can( 'manage_options' ) || $main_site == $blog_id ) {
				return;
			}
			
			if( ! $psts->is_pro_site( $blog_id ) ) {
				switch_to_blog( $main_site );
				wp_safe_redirect( get_permalink( $this->_page_to_redirect ) );
			}

		}

	}

	if ( ! function_exists( 'wpmudev_ps_disable_free_front' ) ) {

		function wpmudev_ps_disable_free_front() {
			return WPMUDEV_PS_Disable_Free_Front::get_instance();
		}

		add_action( 'plugins_loaded', 'wpmudev_ps_disable_free_front', 10 );

	}

}