Add inventory images for tubes.

tubes.lua

@@ -157,7 +157,7 @@ for zp = 0, 1 do
 	if tname ~= "000000" then
 		tgroups = {snappy=3, tube=1, not_in_creative_inventory=1}
 		tubedesc = desc.." ("..tname..")... You hacker, you."
-		iimg=nil
+		iimg=plain_textures[1]
 		wscale = {x=1,y=1,z=1}
 	else
 		tgroups = {snappy=3, tube=1}
@@ -464,4 +464,4 @@ minetest.register_abm({nodenames={"group:sand_tube"},interval=1,chance=1,
 	end})
 
 modpath=minetest.get_modpath("pipeworks")
-dofile(modpath.."/teleport_tube.lua")
+dofile(modpath.."/teleport_tube.lua")