Alpstein/route-me — pull request #88 by incanus (2012-07-23 19:37:58+00:00)
Title: fix crash when attempting at add nil tileSource

Does what it says on the tin. I've seen this sort of thing in relation to [something like this](https://github.com/mapbox/mapbox-me/blob/master/MapBox%20Me/MBMViewController.m#L49) where a network-fetch-based tile source comes back empty. While this is developer error for not providing an online/offline check and/or alternative layer, avoiding a crash would be nice, too.