What application uses the rstf://scheduleState=1 URL scheme? For some time now (perhaps since the 10.13.3 upgrade of MacOS) I have been waking my Mac up from sleep and seeing between 3 and 7 cascading dialogue boxes with the error message:

"Unable to find an application to open URL rstf://?scheduleState=1, with "Ok, Cancel, Find Application" buttons.

I have searched everywhere and cannot find any reference to rstf or scheduleState (there some scheduleState stuff, but it seems unrelated).

Does anyone know how I can find and kill what is trying to open this URL?

Presumably the job is scheduled somewhere, but crontab just has a SuperDuper entry and I've since deleted SuperDuper.
You can use the RCDefaultApp preference pane to see a list of all URL schemes provided by applications on your system.

These supported URL schemes are defined in the Info.plist of an app, and registered collectively by macOS.

Sadly, I do not have any apps installed that handle rstf:// but please let us know when you find out what it is!