The DataItem Property is Empty

The DataItem property is available only during data binding in the NodeDataBound event:

protected void RadTreeView1_NodeDataBound(object o, RadTreeNodeEventArgs e)
{
    DataRowView dataSourceRow = (DataRowView)e.Node.DataItem;
}

Protected Sub RadTreeView1_NodeDataBound(ByVal o As Object, ByVal e As RadTreeNodeEventArgs)
    Dim dataSourceRow As DataRowView = CType(e.Node.DataItem, DataRowView)
End Sub

You should not use it elsewhere. You can use the Value property or Custom Attributes instead.