500 error after upgrading from 5.0 to 5.4 and adding empty functions.php

Question

I've finally managed to upgrade a WP site from v5.0 to v5.4. The only issue left is when I create an empty functions.php file I get 500 error.

Does someone have an idea why is that? Thanks.

Update: My goal is to place the following into functions.php:

function my_acf_init() {
  acf_update_setting('google_api_key', $_ENV['GOOGLE_MAPS_API_KEY']);
}

add_action('acf/init', 'my_acf_init');

Answer 1

I've managed to print the error message by adding the following to the functions.php:

function exception_handler($exception) {
    echo "Uncaught exception: " , $exception->getMessage(), "\n";
}

set_exception_handler('exception_handler');

For some reason logs did not include the error details before. The error is call to undefined function is_blog_installed() which I have no idea why's that. :D