fix Lua errors when adjusting sync in gameplay

BGAnimations/ScreenGameplay underlay/PerPlayer/SubtractiveScoring.lua

@@ -75,6 +75,12 @@ if mods.SubtractiveScoring then
 			-- used to determine if a player has failed yet
 			local topscreen = SCREENMAN:GetTopScreen()
 
+			-- if the player adjusts the sync of the stepchart during gameplay, they will eventually
+			-- reach ScreenPrompt, where they'll be prompted to accept or reject the sync changes.
+			-- Although the screen changes, this Lua sticks around, and the TopScreen will no longer
+			-- have a GetLifeMeter() method.
+			if topscreen.GetLifeMeter == nil then return end
+
 			-- if this is an undesirable judgment AND we can still count up AND it's not a dropped hold
 			if tns == undesirable_judgment
 			and not received_judgment_lower_than_desired