Error when updating Mailster to PHP 8.x

Hey everyone,
I need to update to PHP 8.x but when I do so a error get thrown when I try to add new users to one of my mailing list. Below the error. Is Mailster not fully PHP 8.x ready yet or is the error locally? I updated every single plugin and WordPress to the newest version.

Fehler-Details
==============
Ein Fehler vom Typ E_ERROR wurde in der Zeile 75 der Datei /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/libs/Ip2Country.php verursacht. Fehlermeldung: Uncaught TypeError: method_exists(): Argument #1 ($object_or_class) must be of type object|string, null given in /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/libs/Ip2Country.php:75
Stack trace:
#0 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/libs/Ip2Country.php(75): method_exists(NULL, 'geoip_country_c...')
#1 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/includes/functions.php(555): Ip2Country->get('::92.195.231.13...', 'code')
#2 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/security.class.php(114): mailster_ip2Country()
#3 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/security.class.php(29): MailsterSecurity->verify(Array)
#4 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-includes/class-wp-hook.php(308): MailsterSecurity->verify_subscriber(Array)
#5 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-includes/plugin.php(205): WP_Hook->apply_filters(Array, Array)
#6 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/subscribers.class.php(998): apply_filters('mailster_verify...', Array)
#7 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/subscribers.class.php(1262): MailsterSubscribers->verify(Array, true)
#8 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-content/plugins/mailster/classes/subscribers.class.php(485): MailsterSubscribers->add(Array)
#9 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-includes/class-wp-hook.php(308): MailsterSubscribers->edit_entry('')
#10 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(NULL, Array)
#11 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#12 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-admin/admin.php(237): do_action('load-newsletter...')
#13 /homepages/13/d127679132/htdocs/modernpost/mymail/wp-admin/edit.php(10): require_once('/homepages/13/d...')
#14 {main}
 thrown