private void InnerAddChild(TNode node, int?insertIndex)
{
if (insertIndex < 0)
{
throw new ArgumentOutOfRangeException(nameof(insertIndex));
}
if (!insertIndex.HasValue || insertIndex.Value >= ChildrenList.Count)
{
ChildrenList.Add(node);
}
else
{
ChildrenList.Insert(insertIndex.Value, node);
}
}