Upgrade from 3.0 to 3.1 not working with Custom Logger

Description

I have a custom logging function in .../project/config/app.php. This function in the components array uses Craft::createObject(...) to add a custom Log Target. Craft::createObject(...) uses craft\helpers\App::mailerConfig() somewhere.

In .../project/vendor/craftcms/cms/src/helpers/App.php, function mailerConfig, the $settings variable is null, which triggers $settings = static::mailSettings();. $settings itself is then not null but the variables $settings->transportType, $settings->transportSettings are null. And MailerHelper::createTransportAdapter has no null check.

Beside that error, is there a way to check if the system is in an upgrade process? Otherwise I cannot check the Mailer Config is empty and my upgrade process fails. I manually deactivate the custom logger target, but then I also have to upgrade manually on the production system.

Additional info

• Craft version: 3.1.3

Start in vendor/craftcms/cms/src/helpers/App.php at mailerConfig and trace the Craft::createObject(...) path from the custom logger configuration in project/config/app.php. Inspect MailerHelper::createTransportAdapter and the upgrade lifecycle to determine how null mail settings should be handled or detected. Done means the custom logger no longer blocks the 3.0-to-3.1 upgrade when mailer settings are empty, with coverage for that path.