unable to add node in treeview after selected Item getting an error Index must be within the bounds of the List. Parameter name: index

When i am adding node in treeview after selecting any node some time its adding and some time item not adding and giving an error Index must be within the bounds of the List. Parameter name: index
New node should add after the selected node, when  I am trying to add item to next node which is selected but its adding any where else.. and some time its not geting add
Code i am following is
public void AddWorkflowItem()
        {
            DeletePageNoFromContentTreeView();
            Go to the complete details ...