But wait, there is more!

No...

code/game/objects/structures/door_assembly.dm

@@ -205,6 +205,9 @@
 					if(!mineral)
 						if(istype(G, /obj/item/stack/sheet/mineral) && G.sheettype)
 							var/M = G.sheettype
+							var/mineralassembly = text2path("/obj/structure/door_assembly/door_assembly_[M]")
+							if(!mineralassembly)
+								return
 							if(G.get_amount() >= 2)
 								playsound(src, 'sound/items/crowbar.ogg', 100, 1)
 								user.visible_message("[user] adds [G.name] to the airlock assembly.", \
@@ -214,7 +217,6 @@
 										return
 									to_chat(user, "<span class='notice'>You install [M] plating into the airlock assembly.</span>")
 									G.use(2)
-									var/mineralassembly = text2path("/obj/structure/door_assembly/door_assembly_[M]")
 									var/obj/structure/door_assembly/MA = new mineralassembly(loc)
 									transfer_assembly_vars(src, MA, TRUE)
 							else