in lib/src/authorization_code_grant.dart [280:289]
Future<Client> handleAuthorizationCode(String authorizationCode) async {
if (_state == _State.initial) {
throw StateError('The authorization URL has not yet been generated.');
} else if (_state == _State.finished) {
throw StateError('The authorization code has already been received.');
}
_state = _State.finished;
return await _handleAuthorizationCode(authorizationCode);
}