fix lua error in 2p_Dark_Forecast

data/multiplayer/scenarios/2p_Dark_Forecast.lua

@@ -279,6 +279,9 @@ end)
 --   which unit types get spawned is defined in the 'main_spawn' wml array which is also spawned at prestart
 on_event("new turn", function()
 	local next_spawn = wml.variables["timed_spawn[0]"]
+	if next_spawn == nil then
+		return
+	end
 	if wesnoth.current.turn ~= next_spawn.turn then
 		return
 	end