PHP8 Error

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($function) must be a valid callback, class InboxWebmail_Helper_Upgrade does not have a method “upgrade” in /var/www/vhosts/example.com/httpdocs/wp-includes/class-wp-hook.php:303 Stack trace: #0 /var/www/vhosts/example.com/httpdocs/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters() #1 /var/www/vhosts/example.com/httpdocs/wp-includes/plugin.php(470): WP_Hook->do_action() #2 /var/www/vhosts/example.com/httpdocs/wp-admin/plugins.php(193): do_action() #3 {main} thrown in /var/www/vhosts/example.com/httpdocs/wp-includes/class-wp-hook.php on line 303