fatal error on my website with ongkoskirim

Good morning ,

I have bought the ongkos kirim on your website, maybe ONGKOSKIRIM need an update because not compatibility …When I consult my orders, I cannot see the details and a fatal error appears:

HERE THE MESSAGE I RECEIVED BY EMAIL :

Since WordPress 5.2 there is a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.

In this case, WordPress caught an error with one of your plugins, Plugin Ongkos Kirim.

First, visit your website (https://www.i-rewood.com/) and check for any visible issues. Next, visit the page where the error was caught (https://www.i-rewood.com/wp-admin/post.php?post=6233&action=edit) and check for any visible issues.

Please contact your host for assistance with investigating this issue further.

If your site appears broken and you can't access your dashboard normally, WordPress now has a special "recovery mode". This lets you safely login to your dashboard and investigate further.

https://www.i-rewood.com/log-in-2/?action=enter_recovery_mode&rm_token=E4TIxFA9uwEN7xQEnxnu9p&rm_key=6PbreoMl1gyz7cMgCGNP8E

To keep your site safe, this link will expire in 1 day. Don't worry about that, though: a new link will be emailed to you if the error occurs again after it expires.

When seeking help with this issue, you may be asked for some of the following information:
WordPress version 5.7.1
Current theme: dt-the7 Child (version )
Current plugin: Plugin Ongkos Kirim (version 3.6.1)
PHP version 7.4.15

Error Details
=============
An error of type E_ERROR was caused in line 139 of the file /home/aruvuoe/www/wp-content/plugins/plugin-ongkos-kirim/classes/class-pok-helper.php. Error message: Uncaught Error: Call to a member function get_shipping_country() on null in /home/aruvuoe/www/wp-content/plugins/plugin-ongkos-kirim/classes/class-pok-helper.php:139
Stack trace:
#0 /home/aruvuoe/www/wp-content/plugins/plugin-ongkos-kirim/classes/class-pok-hooks-addresses.php(168): POK_Helper->get_country_session('shipping')
#1 /home/aruvuoe/www/wp-content/plugins/plugin-ongkos-kirim/classes/class-pok-hooks-addresses.php(157): POK_Hooks_Addresses->alter_fields(Array, 'shipping')
#2 /home/aruvuoe/www/wp-includes/class-wp-hook.php(294): POK_Hooks_Addresses->custom_shipping_fields(Array)
#3 /home/aruvuoe/www/wp-includes/plugin.php(212): WP_Hook->apply_filters(Array, Array)
#4 /home/aruvuoe/www/wp-content/plugins/woocommerce/includes/class-wc-countries.php(1514): apply_filters('woocommerce_shi...', Array, 'ID')
#5 /home/aruvuoe/www/wp-content/plugins/woo-checkout-field-editor-pro/includes/utils/class-thwcfd-utils.php(118): WC_Countries->get_address_fields('ID', 'shipping_')
#6 /home/aruvuoe/www/wp-content/plugins