void insertNewNode(@NotNull Node newChild, int childIndex) {
    ThreadingAssertions.assertEventDispatchThread();
    if (children == null) {
      //obsolete type is required by platform superclass
      //noinspection UseOfObsoleteCollectionType
      children = new Vector<>();
    }
    children.insertElementAt(newChild, childIndex);
  }
void insertNewNode(@NotNull Node newChild, int childIndex)