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You are receiving this exception because you are adding an unbound node which does not have an associated row from the underlying data source. To add new nodes to a data bound RadTreeView, you should add records directly to the data source:
private void radButtonAdd_Click(object sender, EventArgs e)
TreeViewTestDataSet.NodeRow row = this.treeViewTestDataSet.Node.NewNodeRow();
row.Name = "New Node";
I hope this will help. Should you have any additional questions, feel free to ask.
the Telerik team