[in_app_purchase] Initialize SKError with correct data type

[vladb99]

Makes sure that SKError.fromJson() gets the correct data type as argument.

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#85534

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[mvanbeusekom]

Thank you for this fix I think the code looks good. The only thing missing is a unit-test that fails using the old code and works correctly with the new updated code.

This might mean that you will need to open up the SKPaymentQueueWrapper._handleObserverCallbacks(MethodCall) method and decorate it with the @visibleForTesting attribute. This would be fine, but make sure you also add the required method documentation. You can use the SKPaymentQueueWrapper. handlePaymentQueueDelegateCallbacks(MethodCall) method as example (example tests can be found in the sk_payment_queue_delegate_api_test.dart file).

[mvanbeusekom]

LGTM, thank you for this contribution.