code: @CheckForNull
      public T newInstanceFromRadioList(JSONObject config) throws FormException {
          if(config.isNullObject())
              return null;    // none was selected
          int idx = config.getInt("value");
          return get(idx).newInstance(Stapler.getCurrentRequest(),config);
      }
docs: Creates a new instance of a {@link Describable}
  from the structured form submission data posted
  by a radio button group.
  @param config Submitted configuration for Radio List
  @return new instance or {@code null} if none was selected in the radio list
  @throws FormException Data submission error