Additional GC Fixes

code/game/objects/effects/effect_system.dm

@@ -33,6 +33,9 @@ would spawn and follow the beaker, even if it is carried or thrown.
 
 	proc/start()
 
+	proc/Destroy()
+		holder = null
+		return ..()
 
 /////////////////////////////////////////////
 // GENERIC STEAM SPREAD SYSTEM

code/modules/mob/living/simple_mob/subtypes/mechanical/mecha/mecha.dm

@@ -57,7 +57,7 @@
 	return ..()
 
 /mob/living/simple_mob/mechanical/mecha/Destroy()
-	qdel(sparks)
+	qdel_null(sparks)
 	return ..()
 
 /mob/living/simple_mob/mechanical/mecha/death()