code: public Object getInComplexType(Object previous, long row) throws IOException {
      if (nextIsNullInComplexType()) {
        return null;
      }
  
      previousRow = row;
  
      return next(previous);
    }
docs: Should be called only from containers (lists, maps, structs, unions) since for these tree
  readers the number of rows does not correspond to the number of values (e.g. there may
  be many more if it contains the entries in lists with more than one element, or much less
  if it is the fields in a struct where every other struct is null)
  
  @param previous
  @return
  @throws IOException