Call to undefined method at checkout

-- HikaShop version -- : 5.0.0
-- Joomla version -- : 4.4.0
-- PHP version -- : 8.2.4
-- Error-message(debug-mod must be tuned on) -- : Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin()

I am in the process of (finally) updating a clients website to Joomla 4. I have made a local (Xampp) clone of the live website and thought I had everything working smoothly. However when I try to place an order though the shop, after hitting the 'finish' button to finalize the order, I get a blank screen with the error message: 0 Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin() (See attached screenshot)

In the administrator back-end of the site I can see that the order has been registered, however I do not receive a confirmation e-mail of this order. When I try to edit the order in the back-end and want to send the customer a notification of the changes, I receive this same error message. (See attached screenshot). Because of this, I think this error message may have something to do with the sending of e-mails from the webshop.

To make sure this error did not have anything to do with my template, I switched to the default Casiopea template, but the issue still persists.

Hi Philip,

Thank you for your quick reply. I figured out what the issue was. The e-mail customizations I made in the old version of Joomla/Hikashop were still present and active under System > Emails. Deleting these made the errors go away.

Thanks for your help!