Fix a bug in the sandworm lair when using shockbolt tiles

git-svn-id: http://svn.net-core.org/repos/t-engine4@3404 51575b47-30f0-44d4-a5cc-537603b46e54

game/modules/tome/class/NicerTiles.lua

@@ -42,7 +42,7 @@ function _M:getTile(name)
 		self.repo[name] = game.zone:makeEntityByName(game.level, "terrain", name)
 		e = self.repo[name]
 	end
-	if e.force_clone then
+	if e and e.force_clone then
 		e = e:clone()
 	end
 	return e