Someone asked help in Symfony slack:

Anyone can help me with this: https://gist.github.com/tmac14/b5ebb4fc723761a44314fc231f53a08c 
--- Typed property App\Controller\Admin\AttachmentController::$slugger must not be accessed 
before initialization

I created this repository to try to reproduce that error, but looks like all works as expected.

1. Clone this repository
2. Run composer install
3. Run symfony serve
4. Open http://localhost:8000 in your browser
5. Profit that you see dumps about those paths and SluggerInterface

MIt