The function is_user_logged_in() is not available at shutdown (plugin context, not theme).

I want my plugin to run only (1) on front-end pages and (2) when the user is not logged in.

This partially works, but it also runs on Divi builder front-end pages, so I need to add !is_user_logged_in:

if (!is_admin()) {
    add_action('shutdown', function () {
        // Do some ob_start() and PHP_Tidy() stuff...
    }, 0);
}

Unfortunately changing my conditional to if (!is_admin() && !is_user_logged_in()) fails with:

Uncaught Error: Call to undefined function is_user_logged_in() in...