I'm sure that if you uninstalled the app, that it also would fix it - since it's not there anymore
Anyway.. all jokes aside. I think the one thing you can try is to, before opening ForkLift 3 for the first time(or just do it it now even), to go to System Preferences and put ForkLift 3 inside the 'Full Disk Access' section by first unlocking System Preferences(because the field where ForkLift is to be placed is greyed out before unlocking) and just drag the ForkLift 3 app over that field. Or click the + button and localte ForkLift in your Applications folder.