Joomla 4: 0 Call to undefined method on End of Checkout

-- url of the page with the problem -- :
-- HikaShop version -- : 4.6.1
-- Joomla version -- : ‎4.1.5
-- PHP version -- : 7.4.30
-- Browser(s) name and version -- : Any
-- Error-message(debug-mod must be tuned on) -- : 0 Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin()

At the final step of checkout, on pressing submit, it throws the following error:
0 Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin()

No order is created.

For testing, you can use this coupon for 99% discount: SYSTEM-TEST-2

I have sent you login details.

Here is the debug grid:

0 Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin()
Call stack
# Function Location
1 () JROOT/media/com_hikashop/mail/order_creation_notification.preload.modified.php:62
2 include() JROOT/administrator/components/com_hikashop/classes/mail.php:362
3 hikashopMailClass->loadEmail() JROOT/administrator/components/com_hikashop/classes/mail.php:31
4 hikashopMailClass->get() JROOT/administrator/components/com_hikashop/classes/order.php:1997
5 hikashopOrderClass->loadOrderNotification() JROOT/administrator/components/com_hikashop/classes/order.php:441
6 hikashopOrderClass->save() JROOT/administrator/components/com_hikashop/classes/order.php:906
7 hikashopOrderClass->createFromCart() JROOT/components/com_hikashop/controllers/checkout.php:671
8 checkoutController->confirm() JROOT/libraries/src/MVC/Controller/BaseController.php:735
9 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/administrator/components/com_hikashop/helpers/helper.php:2382
10 hikashopController->execute() JROOT/components/com_hikashop/hikashop.php:88
11 require_once() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:69
12 Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:71
13 Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:389
14 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/SiteApplication.php:204
15 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/SiteApplication.php:243
16 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:278
17 Joomla\CMS\Application\CMSApplication->execute() JROOT/includes/app.php:63
18 require_once() JROOT/index.php:32

That makes sense. This website is one of the oldest HikaShop installs I have.

Renaming the file did not stop the error.

I ended up deleting all of the customization for the emails.
That is working now.

Thank you.