This recipe improves compatibility for ThemeCo's Cornerstone Editor plugin when editing pages that requires a membership level.

my-pmpro-themeco-pro-cornerstone-editor.php

<?php
/**
 * This recipe improves compatibility for ThemeCo's Cornerstone Editor plugin
 * when editing pages that requires a membership level.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_cornerstone_editor_before_load_preview() {
	if ( defined( 'PMPRO_MEMBER_DIRECTORY_VERSION' ) ) {
		add_filter( 'pmpro_has_membership_level', '__return_true' );
	}
}
add_action( 'cs_before_preview_frame', 'my_pmpro_cornerstone_editor_before_load_preview' );