Not sure if this is only related to WordPress... getting below output when running a CLI command. Started happening with 3.13.2. Maybe caused by #15472 ? Not sure...

It's just a notice though. Things generally still seems to work.

Notice: Trying to access array offset on value of type null in core/Session/SessionAuth.php on line 63

Call Stack:
0.0010 397688 1. {main}() console:0
0.9315 47844424 2. Piwik\Console->run() console:32
0.9343 48033128 3. Piwik\Console->doRun() vendor/symfony/console/Symfony/Component/Console/Application.php:126
0.9343 48033128 4. Piwik\Console->doRunImpl() core/Console.php:93
1.0943 57948336 5. Piwik\Access::doAsSuperUser() core/Console.php:141
1.0954 57967824 6. Piwik\Console->Piwik{closure:core/Console.php:139-141}() core/Access.php:644
1.0954 57968200 7. call_user_func:{core/Console.php:140}() core/Console.php:140
1.0954 57968200 8. Piwik\Console->doRun() core/Console.php:140
1.0955 57968472 9. Piwik\Console->doRunCommand() vendor/symfony/console/Symfony/Component/Console/> Application.php:195
1.0956 57968472 10. Piwik\Plugins\WordPress\Commands\GenerateCoreAssets->run() vendor/symfony/console/Symfony/> Component/Console/Application.php:874
1.0957 57970664 11. Piwik\Plugins\WordPress\Commands\GenerateCoreAssets->execute() vendor/symfony/console/> Symfony/Component/Console/Command/Command.php:257
1.2942 58002656 12. Piwik\FrontController->init() /Users/thomassteur/Development/wpsingle/wp-content/plugins/> matomo/plugins/WordPress/Commands/GenerateCoreAssets.php:57
1.3136 58568120 13. Piwik\FrontController->makeAuthenticator() core/FrontController.php:406
1.3139 58568120 14. Piwik\Session\SessionAuth->getLogin() core/FrontController.php:701