Deleting payment method breaks order payments in back office

Problem/Motivation

If a customer deletes a payment method, a fatal error occurs when the canCapturePayment() method is called when accessing back-office functions such as viewing payments on the order or attempting a refund.

Error is:

Error: Call to a member function canCapturePayment() on null in Drupal\commerce_stripe\Plugin\Commerce\PaymentGateway\StripePaymentElement->canCapturePayment() (line 1453 of /var/www/html/docroot/modules/contrib/commerce_stripe/src/Plugin/Commerce/PaymentGateway/StripePaymentElement.php)

(Ignore the line number as this is on 2.0.1)

Steps to reproduce

1. Check out and complete an order as a customer
2. Delete the saved payment method used
3. Log in as an admin and go to /admin/commerce/orders/{order_id}/payments
4. Error is encountered as above

Proposed resolution

Extend the checking to ensure that, we don't call the canCapturePayment() method on a PaymentType that is NULL

Remaining tasks

Requires review