Did you check “Pass-by-reference” when you created your TriggerBox input? Unchecking this fixed it for me, however you lose the fuctionality of it. You can just manually do the pass by reference by outputting TriggerBox and setting it from the blueprint that called the event.
Edit: I didn’t notice this at first, but the warning doesn’t change any funcitonality as it still works. I’ll just leave this up since personally warning messages bug the hell out of me.