Pages added to admin menu are restricted for anyone except of admins

I had to add a page to the admin part via

add_management_page( 'Vertr?ge anzeigen', 'Vertr?ge anzeigen', 'edit_posts', 'show_contracts', 'gr_show_contracts' );

This page should be accessible for editors. Unfortunately it is only visible for admin. If an editor tries to access that page, he get’s an error message “Cannot load show_contracts”.

Reason: Plugin Restrict Content by Role hardcodes allowed capabilities in line 43 of class module class.AdminAccess.php:

if( is_admin() && ! current_user_can( 'manage_options' ) ) { … }

Changing the capabilities above to “edit_post” solves the problem.

But: this should be done in the child theme and this does not work.

I tried the following in my functions.php:

function gr_admin_access() { 
	if (class_exists( 'mkdo\restrict_content_by_role\AdminAccess' )) {
		get_template_part('gr-admin-access');
		$gr_admin_access = new GR_AdminAccess;
	}
}
add_action('init', 'gr_admin_access');

and in gr-admin-access.php:

<?php

use mkdo\restrict_content_by_role\AdminAccess;

class GR_AdminAccess extends AdminAccess {

	public function run() {
remove_action( 'admin_init', array( '\mkdo\restrict_content_by_role\AdminAccess', 'get_excluded_posts' ) );
	add_action( 'admin_init', array( $this, 'get_excluded_posts' ) );
	}

	/**
	 * Check if user has access
	 */
	public function get_excluded_posts() {

		if( is_admin() && ! current_user_can( 'edit_posts' ) ) {
		...
	}
...
}

The new class is invoked, but I still get the error “Cannot load …” whereas if I change that line in the original class, it works.

What am I doing wrong? Any help would me very much appreciated.