Symfony version(s) affected: 5.0.x

Description

we are in the process of migrating from 4.4 to 5.0
however some of the registered event listeners, don't return a priorty - aka null

this happens around here:

https://sourcegraph.com/github.com/symfony/symfony@20bf17f6ad38011f90cfe9b2dc7c30c8901163e5/-/blob/src/Symfony/Component/EventDispatcher/EventDispatcher.php#L117

as the $this->listeners[$eventName] is nested one more times.

it results in a final exception:

due to https://sourcegraph.com/github.com/symfony/symfony@20bf17f6ad38011f90cfe9b2dc7c30c8901163e5/-/blob/src/Symfony/Component/EventDispatcher/Debug/TraceableEventDispatcher.php#L55 - beeing type hinted with int

introduced with: 2bc9472

my event definition:

services:
    krn.demo:
        class: App\EventSubscriber\Demo
        public: false
        shared: false
        tags:
          - { name: kernel.event_listener, event: krn.demo, method: onEvent }

How to reproduce

clone: https://github.com/hjanuschka/demo

composer install
 bin/console debug:event-dispatcher krn.demo

where it should show 0

Possible Solution

return 0; instead of return null in getListenerPriority

PR: #35260

any help?