code: DbxAuthFinish withUrlState(/*@Nullable*/ String urlState) {
          if (this.urlState != null) {
              throw new IllegalStateException("Already have URL state.");
          }
  
          DbxAuthFinish result =  new DbxAuthFinish(accessToken, expiresIn, refreshToken, userId,
                  teamId, accountId, urlState);
          result.setIssueTime(issueTime);
  
          return result;
      }
docs: State is not returned from /oauth2/token call, so we must
  append it after the JSON parsing.
  
  @param urlState Custom state passed into /oauth2/authorize