docs: public static AbstractButton getAt(ButtonGroup self, int index) {
          int size = self.getButtonCount();
          if (index < 0 || index >= size) return null;
          Enumeration buttons = self.getElements();
          for (int i = 0; i <= index; i++) {
              AbstractButton b = (AbstractButton) buttons.nextElement();
              if (i == index) return b;
          }
          return null;
      }