PHP Error

How to fix this?

Notice: Undefined index: age_gate in /var/www/vhosts/domain.de/httpdocs/wp-content/plugins/age-gate-user-registration/public/class-age-gate-user-registration-woocommerce.php on line 127

Fatal error: Uncaught TypeError: Argument 1 passed to GUMP::is_valid() must be of the type array, null given, called in /var/www/vhosts/domain.de/httpdocs/wp-content/plugins/age-gate-user-registration/public/class-age-gate-user-registration-woocommerce.php on line 130 and defined in /var/www/vhosts/domain.de/httpdocs/wp-content/plugins/age-gate/vendor/wixel/gump/gump.class.php:99 Stack trace: #0 /var/www/vhosts/domain.de/httpdocs/wp-content/plugins/age-gate-user-registration/public/class-age-gate-user-registration-woocommerce.php(130): GUMP::is_valid() #1 /var/www/vhosts/domain.de/httpdocs/wp-includes/class-wp-hook.php(287): Age_Gate_User_Registration_WooCommerce->wooc_validate_extra_register_fields() #2 /var/www/vhosts/domain.de/httpdocs/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters() #3 /var/www/vhosts/domain.de/httpdocs/wp-includes/plugin.php(478): WP_Hook->do_action() #4 /var/www/vhosts/domain.de/httpdocs/wp-content/plugins/woocommerce/ in /var/www/vhosts/domain.de/httpdocs/wp-content/plugins/age-gate/vendor/wixel/gump/gump.class.php on line 99