Trying to access array offset on value of type null with php 7.4

[vasilvestre]

Describe the bug
When you try to access admin, using 2.x-dev, this error is shown Trying to access array offset on value of type null
This is a notice.

To Reproduce
use php 7.4
composer require symfony/flex
composer require admin
go on /admin

[javiereguiluz]

In case it's useful, I'm adding the references you mentioned on Symfony Slack:

https://wiki.php.net/rfc/notice-for-non-valid-array-container
and a related issue on Doctrine annotations: doctrine/annotations#273

[vasilvestre]

And here's the PHP SRC PR : php/php-src#4386