onFinalizeSubscriptionFinished expects the Tp::PendingOperation to have a property set with the current Contact In one of the two places that it is called from this is the case, however when called from onAuthorizePresencePublicationFinished this is not set. This crash will only ever appear if the operation failed which is why we haven't seen it.
Fixed in 3b0fb7c2a8e7ae57819915aafa47aa2321cd732c