`NullReferenceException` error when trying to get a recipient based on `SubjectKeyIdentifier`

[rosco12345]

We recently tried to update from version 2.3.1 to 2.4.0

This caused our encryption tests to break. I've produced a minimal example here. It's simply a test that encrypts a string then decrypts it, asserting that the original plaintext and the decrypted plaintext are the same.

The test in that example passes with version 2.3.1 and fails with a null reference error when using GetFirstRecipient().

I wonder if this could be related to changes made in a35474d