While running the API tests for one of the plugins I received a bunch of the following PHP warning:

PHP Deprecated:  urlencode(): Passing null to parameter #1 ($string) of type string is deprecated in matomo/vendor/matomo/matomo-php-tracker/MatomoTracker.php on line 1931

The warnings appear to be happening because a line of the tracker is trying to encode the referrer URL when there isn't one.

PHP: 8.1.2
Matomo: 4.12.3
MultiChannelConversionAttribution: 4.1.0