Fix and test for a regression introduced with #4243: As the `PayPaymentRequest` action allows anonymous access, the `CookieAuthorizationHandler` isn;t run and hence the `GetCurrentStore` returns `null`.
This leads to an exception when creating the invoice. Store needs to be fetched seperately - like [before](4bbc7d9662 (diff-bdc264670a171e862d09fdf1a1c9f3ca14b41982a3c4c8e66d4f780cdde9f21dL241)).
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