I’m experiencing the exact same issue as the one reported at Custom Selector error unique 'id' property. Since it’s a quite old thread I’ve decided to open a new one. It’s the exact same issue with the error message showing up in the Custom Selector dropdown, but now in Content Studio 3.3.4 / XP 7.7.2.
Logs have been added to the service used in the Custom Selector and the data structure being returned in the body object looks all good. Considering it’s a solution that’s been running stable for a few years now and we’ve only been experiencing such error recently I wonder if there could be any relation to the version of Content Sutdio (3.3.4).
The error message will only show up sometimes, with no specific pattern. No error/exception is actually issued in the output logs.