I had a Path Of Knives on my opponent, who had 2 hp. He used Nimble Strike and died (from path of knives) before he could select a target for the attack portion of nimble strike. Then the game got stuck with him trying to select a target, which he couldn't. We were both still able to access to the in-game menu, and he resigned (his timer would have killed him anyway, since he was stuck and it was counting down).
BATTLE LOG: Scenario=MP Celestial Dojo,Room=Lord Feleran vs Flaxative,RoomID=90608,Msg=Player Lord Feleran must select targets That was my last message in console. Nimble Strike was still in my inventory as if I had only stepped but I couldn't finish it because that char was dead.
I just tested this and, while Nimble Strike stayed waiting to be played, I could click Done to continue. Is it possible that you just didn't notice the Done button there?
I do know that this won't cause a crash when the dead person is AI-controlled. In that case, the AI would be hitting "Done" silently, so all of the above is consistent.
Confirmed, my opponent died using a step attack with PoK today and they found the "Done" button and the game ended.