How to allow users to select joints from Rig Tree (like Group Joints SOP)?

[raincole 2]

I'm trying to make a parameter action button that pops up Rig Tree Pane and allows user to select joints from it. Just like the Group Joints SOP:
 

 

I tried to copy this parameter from Group Joint SOP. But it doesn't work. The Rig Tree Pane pops up, but when I click any joint on it, it just closes itself without updating the parameter's value.

My HDA is attached.

What did I do wrong?

rainc_subnet1.hdalc