Fix checkout flow activity close on error.

app/src/main/java/org/thoughtcrime/securesms/backup/v2/ui/subscription/MessageBackupsFlowFragment.kt

@@ -273,4 +273,8 @@ class MessageBackupsFlowFragment : ComposeFragment(), InAppPaymentCheckoutDelega
   override fun navigateToDonationPending(inAppPayment: InAppPaymentTable.InAppPayment) {
     // TODO [message-backups] What do? Are we even supporting bank transfers?
   }
+
+  override fun exitCheckoutFlow() {
+    requireActivity().finishAfterTransition()
+  }
 }