I’m a Script Debugger user. Using local makes variables visible within the Debugger.
Unfortunately, your script failed as well.
Rebooting macOS fixed the problem. 
I thought, that is just true for windows. Two hours for nothing. Thanks a lot for helping me out at identifying the real issue! Having a known working example as always good while troubleshooting.
Maybe something was cached by the AppleScript runner stuff, while I try to get a smart rule running using https://discourse.devontechnologies.com/t/smart-rule-fails-only-when-triggered-as-a-smart-rule-can-t-get-name-of-class-dtcn-id-1296597-of-class-dtkb-id-2/70691/28.