docs: public Source retrieveSourceSynchronous(
              @NonNull @Size(min = 1) String sourceId,
              @NonNull @Size(min = 1) String clientSecret,
              @Nullable String publishableKey)
              throws AuthenticationException,
              InvalidRequestException,
              APIConnectionException,
              APIException {
          String apiKey = publishableKey == null ? mDefaultPublishableKey : publishableKey;
          if (apiKey == null) {
              return null;
          }
          return mApiHandler.retrieveSource(sourceId, clientSecret, apiKey, mStripeAccount);
      }