Invalid selector by UI Explorer

Today, I tried to select my game windows (just call ‘Mu Online’) from UI Explorer, unfortunately, it returns nothing. Ironically, visual tree still show ‘Mu Online’ as an element, so I tried select ‘Mu Online’ element and click the ‘Validate selector’ button and the message “The current selector is empty! Please indicate the target object on the screen” is appeared, I also try indicate ‘Mu Online’ on the screen but nothing is selected.
The question: are there anyway to select my ‘Mu Online’ windows? I think developers has hidden ‘Mu Online’ from third party.