Leaving this here in case it affects others.

App not starting under Catalina: generates blank window and does nothing.  Is possible to quit.

Michaels-MacBook-Pro:MacOS mcroft$ ./Thunderbeam 
objc[920]: Class AVAudioPlayer is implemented in both /System/Library/Frameworks/AVFoundation.framework/Versions/A/Frameworks/AVFAudio.framework/Versions/A/AVFAudio (0x7fff9389c7a0) and /Users/mcroft/Downloads/Thunderbeam-2.app/Contents/MacOS/./Thunderbeam (0x101a86de0). One of the two will be used. Which one is undefined.
objc[920]: Class AVAudioSession is implemented in both /System/Library/Frameworks/AVFoundation.framework/Versions/A/Frameworks/AVFAudio.framework/Versions/A/AVFAudio (0x7fff9389c840) and /Users/mcroft/Downloads/Thunderbeam-2.app/Contents/MacOS/./Thunderbeam (0x101a86e58). One of the two will be used. Which one is undefined.
2019-07-21 22:07:54.759 Thunderbeam[920:14831] NSSoftLinking - The function 'SLSIsSuppressedByScreenTime' can't be found in the (null) framework.
2019-07-21 22:07:55.567 Thunderbeam[920:14831] TBSave Init
2019-07-21 22:07:55.567 Thunderbeam[920:14831] TBAudio init
2019-07-21 22:07:55.608 Thunderbeam[920:14831] *** +[NSString stringWithCString:encoding:]: NULL cString

Google provides possible StackOverflow reference: https://stackoverflow.com/questions/57076587/nssoftlinking-the-function-slsissup...

Attempting to decrypt FileVault on HD, will try again after decrypt complete (2 hours).

Update: Undoing and re-doing FileVault does not help.  App does nothing on Catalina after that 07:55.608 error.

will keep trying as they and you update...

FWIW, still failing in Big Sur...

fixed in 1.11