fix crash when payment.productIdentifier is nil (#226)
fix crash when payment.productIdentifier is nil (#226)

* updated usage of productIdentifier in purchasePackage so that we only use the one that's guaranteed not to be nil

* added an extra nil check to prevent a different crash
RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios