Thank you for writing.
I created a sample project demonstrating how you can remove the tree from the popup. In order to assign the new popup, you would need to handle the GroupDescriptorElementCreating
I hope this helps. Should you have further questions please do not hesitate to write back.
Telerik by Progress
Check out the Windows Forms project converter, which aids the conversion process from standard Windows Forms applications written in C# or VB to Telerik UI for WinForms. For more information check out this blog post
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