suggested changes

code/modules/mob/living/carbon/human/species/shadow.dm

@@ -39,7 +39,7 @@
 /datum/action/innate/shadow/darkvision/Activate()
 	var/mob/living/carbon/human/H = owner
 	if(!H.vision_type)
-		H.set_sight(new /datum/vision_override/nightvision)
+		H.set_sight(/datum/vision_override/nightvision)
 		to_chat(H, "<span class='notice'>You adjust your vision to pierce the darkness.</span>")
 	else
 		H.set_sight(null)

code/modules/mob/mob.dm

@@ -1357,9 +1357,9 @@ GLOBAL_LIST_INIT(slot_equipment_priority, list( \
 	sync_lighting_plane_alpha()
 
 /mob/proc/set_sight(datum/vision_override/O)
-	if(vision_type)
-		qdel(vision_type) //qdel the old vision_type unless it is null.
-	vision_type = O
+	QDEL_NULL(vision_type)
+	if(O) //in case of null
+		vision_type = new O
 	update_sight()
 
 /mob/proc/sync_lighting_plane_alpha()