Thank you for writing.
Currently, to display the blank items in the filter, you can manually add item to the treeview in the filter popup. This can be done in the FilterPopupInitialized event:
sender, FilterPopupInitializedEventArgs e)
RadListFilterPopup popup = e.FilterPopup
Also we have a feature request for this in our Feedback Portal
. You can track the item for status changes and add your vote for it here
If you have any questions, please do not hesitate to contact us.
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