[UPMERGE] 1.0 -> 2.0 (#134)

This PR has been generated automatically.
For more details see
[upmerge_pr.yaml](/Sylius/MolliePlugin/blob/1.0/.github/workflows/upmerge_pr.yaml).

**Remember!** The upmerge should always be merged with using `Merge pull
request` button.

In case of conflicts, please resolve them manually with usign the
following commands:
```
git fetch upstream
gh pr checkout <this-pr-number>
git merge upstream/2.0 -m "Resolve conflicts between 1.0 and 2.0"
```

If you use other name for the upstream remote, please replace `upstream`
with the name of your remote pointing to the `Sylius/MolliePlugin`
repository.

Once the conflicts are resolved, please run `git merge --continue` and
push the changes to this PR.

Author: TheMilek

src/Validator/Constraints/PaymentMethodMollieChannelUniqueValidator.php

@@ -113,8 +113,11 @@ private function isTheSameChannel(Collection $newChannels, Collection $paymentMe
 
     private function isMolliePaymentMethod(PaymentMethodInterface $paymentMethod): bool
     {
-        /** @var GatewayConfigInterface $gateway */
+        /** @var GatewayConfigInterface|null $gateway */
         $gateway = $paymentMethod->getGatewayConfig();
+        if (null === $gateway) {
+            return false;
+        }
 
         return true === in_array(
             $gateway->getFactoryName(),