Website backend super slow, getting 502 error and AJAX: Warning

Currently having some issues with the backend of our website that has been really so slow sometimes it even gives the 502 loading error. I installed Query monitor and got the following from the browser console can anybody help diagnose the problem, please.

I’m a beginner when it comes to coding and websites etc.

query-monitor.js?ver=1661252892:372 PHP Errors in Ajax Response
query-monitor.js?ver=1661252892:382
{key: ’21fe29aee1711a6c432ac181376a5aa1′, type: ‘warning’, message: ‘foreach() argument must be of type array|object, null given’, file: ‘wp-admin/includes/plugin.php’, line: 1779, …}
component: “Theme”
file: “wp-admin/includes/plugin.php”
key: “21fe29aee1711a6c432ac181376a5aa1”
line: 1779
message: “foreach() argument must be of type array|object, null given”
stack: (3) [‘remove_menu_page()’, ‘Nine3\\S\\B\\U\\remove_comment_support()’, “do_action(‘admin_init’)”]
type: “warning”
Prototype: Object

Any help would be much appreciated.