docs: @UiThread
      public void notifyChildRemoved(int parentPosition, int childPosition) {
          int flatParentPosition = getFlatParentPosition(parentPosition);
          ExpandableWrapper<P, C> parentWrapper = mFlatItemList.get(flatParentPosition);
          parentWrapper.setParent(mParentList.get(parentPosition));
  
          if (parentWrapper.isExpanded()) {
              mFlatItemList.remove(flatParentPosition + childPosition + 1);
              notifyItemRemoved(flatParentPosition + childPosition + 1);
          }
      }