Treeview小弟我选的就是树根啊 如何还会报错呢

Treeview我选的就是树根啊 怎么还会报错呢

C# code

 private void tvMedicines_NodeMouseDoubleClick(object sender, TreeNodeMouseClickEventArgs e)        {                                   MessageBox.Show(tvMedicines.SelectedNode.Parent.Text);}