Ciao,
my matomo_logger_message is full of this.
WARNING HeatmapSessionRecording[2018-12-06 14:06:15 UTC] [55319] /var/www/html/piwik/plugins/HeatmapSessionRecording/VisitorDetails.php(60): Warning - array_key_exists() expects parameter 2 to be array, null given - Matomo 3.6.1 - Please report this message in the Matomo forums: https://forum.matomo.org (please do a search first as it might have been reported already)
any ideas?