I am having this exact same issue - have never yet logged into studio - so can't have set the upgrade channel to Beta.. But interestingly in the stack trace I see... (yet I never ran update to 13)

#6 /home/httpd/html/xxxxxxx.com/public_html/tmp/patch_12.1.0_13.0.0-A1/UNA-Patch-v.12.1.0-to-v.13.0.0 in /home/httpd/html/xxxxxxx.com/public_html/modules/boonex/protean/data/template/system/scripts/BxTemplConfig.php on line 425"