public ArrayList<T> getModifiableList(AbstractPropertyContainer container) {
    final ArrayList<T> modifiableList;
    final List<T> list = get(container);
    if (list instanceof ArrayList) {
      modifiableList = (ArrayList<T>)list;
    }
    else {
      modifiableList = new ArrayList<>(list);
      set(container, modifiableList);
    }
    // remove nulls
    for (int i = modifiableList.size() - 1; i >= 0; --i) {
      if (modifiableList.get(i) == null) {
        modifiableList.remove(i);
      }
    }
    return modifiableList;
  }
public ArrayList<T> getModifiableList(AbstractPropertyContainer container)