I have the same exact problem as the post above. I am also on Mojave.
When I force quit, if I hit report I am given a crash dump I can post if that is helpful.
I checked the logs in "~/LIbrary/Logs/Time Sync/Island Rum" and there's barely anything. Just this:
Mono path[0] = '/Applications/Island Rum.app/Contents/Resources/Data/Managed'
Mono config path = '/Applications/Island Rum.app/Contents/MonoBleedingEdge/etc'
Obtained 32 stack frames.