This dialog is typically invoked from the Column Chooser in ListView or the layout customization form in DetailView.
Starting with v15.2.10, we have added the OK button, renamed the Close button to Cancel, and enabled multi-selection in the tree list within the Object Model dialog.
The Add button adds columns but does not close the dialog. You can also double-click an item to add it.
The OK button applies changes and closes the dialog.
The Cancel button closes the dialog and does not apply changes.
I hope your end-users find this minor thing useful.