code: public <T extends Widget & Checkable> boolean check(int checkableIndex) {
          List<T> children = getCheckableChildren();
          T checkableWidget = children.get(checkableIndex);
          return checkInternal(checkableWidget, true);
      }
docs: Checks the widget by index
  @param checkableIndex The index is in the range from 0 to size -1, where size is the number of
  Checkable widgets in the group. It does not take into account any
  non-Checkable widgets added to the group widget.
  @return {@code True} if {@code checkableWidget} is a child of this {@code CheckableGroup} and
  was not already checked; {@code false} otherwise.