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Issue Description:

We have received these crash logs from a single user. They happened 8 times in a row, so it looks fairly consistent. My assumption is that the underlying cocoa string is (somehow) NSNull, and _bridgeCocoaString fails to handle this. I've attached one of the crash logs (the stack traces are identical on all of them).

Things of notice:

Device was running 13.3.

The last function in the stack trace that we control (makePurchaseInfo) has only two calls to something that returns a String before dispatching code using a closure that is not listed in the stack trace:

1. A call to another class we own that reads the receipt using Bundle.main.appStoreReceiptURL and that eventually returns the data as receiptData.base64EncodedString(). However this class name is not in the stack trace, so I find it unlikely to be this.

2. SKPaymentTransaction.payment.productIdentifier

My belief is that one of these things pointed to a NSNull instance instead of something _bridgeCocoaString understands. That's as much as I could gather from my code and from quickly glancing at StringBridge.swift in the stdlib folder of the Swift repo.

I'm ready to provide more info in case it is needed.