code: @Nullable
      public Source createSourceSynchronous(
              @NonNull SourceParams params,
              @Nullable String publishableKey)
              throws AuthenticationException,
              InvalidRequestException,
              APIConnectionException,
              APIException {
          String apiKey = publishableKey == null ? mDefaultPublishableKey : publishableKey;
          if (apiKey == null) {
              return null;
          }
          return mApiHandler.createSource(params, apiKey, mStripeAccount);
      }
docs: Blocking method to create a {@link Source} object.
  Do not call this on the UI thread or your app will crash.
  
  @param params a set of {@link SourceParams} with which to create the source
  @param publishableKey a publishable API key to use
  @return a {@link Source}, or {@code null} if a problem occurred
  @throws AuthenticationException failure to properly authenticate yourself (check your key)
  @throws InvalidRequestException your request has invalid parameters
  @throws APIConnectionException failure to connect to Stripe's API
  @throws APIException any other type of problem (for instance, a temporary issue with
  Stripe's servers