Dialog Toolkit script fails after minor edit

I’m using the method proposed by Nigel here to have Dialog Toolkit dialogs display with scripts run from the Scripts Menu. Basically, the dialog code is put into a handler in an applet, and the applet is then run by the Scripts Menu script. It has worked well for some time.
I set these applets to run as agents - they don’t show a menu bar or a Dock icon.

With one of them I missed doing that, so a few days ago I added the relevant key to its Info.plist. And it stopped displaying the dialog. It runs, but no dialog. Removing the key did not make it work again. When I run the calling script from Script Editor it works as intended.

Any ideas to get this going again? Some setting in System Preferences? I fiddled with some of them, without success. I’m still on 10.15.