Wordpress error PHP 7.4 - Warning: Illegal string offset 'content'

I'm working with an old wordpress theme called Blessing. My provider recommended to upgrade my server php version to 7. 4 and the website produced error message saying Warning: Illegal string offset 'content' in admin/applications/shortcodes/index.php(87) : eval()'d code on line 3

Warning: Cannot assign an empty string to a string offset in /admin/applications/shortcodes/index.php(87) : eval()'d code on line 3.
<?php
Thememakers_Application_Shortcodes::register();
add_action('admin_head', 'thememakers_app_shortcodes_admin', 1);