I've been using stellar-dotnet-sdk for months; today, I get an error while trying to instantiate an AccountResponse object.
AccountResponse.cs, line 147 (AccountId = accountId ?? throw new ArgumentNullException(nameof(accountId), "accountId cannot be null");). My code hasn't changed - I see this error when hitting both the Test and Prod network.
Something is happening while it is trying to parse the JSON returned from horizon-testnet.stellar.org/accounts/…. As I step through the code, the returned JSON looks fine - but the accountID being passed into the Signer object constructor is null (haven't stepped back further to see the root of all of this).