FATAL error in 3.49493

 An error of type E_ERROR was caused in line 124 of the file /home/www/wordpress/wp-content/plugins/full-site-editing/help-center/class-help-center.php. Error message: Uncaught Error: Class "Jetpack_Options" not found in /home/www/wordpress/wp-content/plugins/full-site-editing/help-center/class-help-center.php:124
Stack trace:
#0 /home/www/wordpress/wp-content/plugins/full-site-editing/help-center/class-help-center.php(113): A8C\FSE\Help_Center->get_current_site()
#1 /home/www/wordpress/wp-content/plugins/full-site-editing/help-center/class-help-center.php(240): A8C\FSE\Help_Center->enqueue_script()
#2 /home/www/wordpress/wp-includes/class-wp-hook.php(308): A8C\FSE\Help_Center->enqueue_wp_admin_scripts('plugins.php')
#3 /home/www/wordpress/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(NULL, Array)
#4 /home/www/wordpress/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#5 /home/www/wordpress/wp-admin/admin-header.php(118): do_action('admin_enqueue_s...', 'plugins.php')
#6 /home/www/wordpress/wp-admin/plugins.php(605): require_once('/home/klubterre...')
#7 {main}
  thrown

I was able to fix the error by tweaking the code in www/wordpress/wp-content/plugins/full-site-editing/help-center/class-help-center.php lines 126-152.

Original code:

/**
   * Get current site details.
   */
  public function get_current_site()
  {
    $site = \Jetpack_Options::get_option("id");
    $logo_id = get_option("site_logo");
    $products   = wpcom_get_site_purchases();
    $at_options = get_option("at_options");
    $plan       = array_pop(
    	array_filter(
    		$products,
    		function ( $product ) {
    			return 'bundle' === $product->product_type;
    		}
    	)
    );

    return [
      "ID" => $site,
      "name" => get_bloginfo("name"),
      "URL" => get_bloginfo("url"),
      "plan" => [
        "product_slug" => $plan->product_slug,
      ],
      "is_wpcom_atomic" => boolval($at_options),
      "jetpack" => true === apply_filters("is_jetpack_site", false, $site),
      "logo" => [
        "id" => $logo_id,
        "sizes" => [],
        "url" => wp_get_attachment_image_src($logo_id, "thumbnail")[0],
      ],
    ];
  }

Tweaked code:

/**
   * Get current site details.
   */
  public function get_current_site()
  {
    $site = \Jetpack_Options::get_option("id");
    $logo_id = get_option("site_logo");
    //$products   = wpcom_get_site_purchases();
    $at_options = get_option("at_options");
    // $plan       = array_pop(
    // 	array_filter(
    // 		$products,
    // 		function ( $product ) {
    // 			return 'bundle' === $product->product_type;
    // 		}
    // 	)
    // );

    return [
      "ID" => $site,
      "name" => get_bloginfo("name"),
      "URL" => get_bloginfo("url"),
      "plan" => [
        "product_slug" => "basic", //$plan->product_slug,
      ],
      "is_wpcom_atomic" => boolval($at_options),
      "jetpack" => true === apply_filters("is_jetpack_site", false, $site),
      "logo" => [
        "id" => $logo_id,
        "sizes" => [],
        "url" => wp_get_attachment_image_src($logo_id, "thumbnail")[0],
      ],
    ];
  }

Commenting out $products = wpcom_get_site_purchases(); and the $plan = array_pop( […] ); prevents the crash, and returning "product_slug" => "basic", //$plan->product_slug, in the response array ensures that the site works properly.