Tilemap.fill doesn't work with blank layers

[DrHackenstein]

If a blankLayer was created, fill produces a Uncaught TypeError: Cannot set property 'index' of null. Looks like fill can't handle that blankLayers are filled with 'null' values. That's kinda strange and probably not how it's supposed to be. Filling a blank layer has to be the most useful application for fill 😃

The error was also mentioned in these threads:
http://www.html5gamedevs.com/topic/5256-replacing-tile-in-tilemap-cant-read-index-of-null/
http://www.html5gamedevs.com/topic/3942-cant-fill-my-tilemap/

As far as I can tell putTile is fixed and can be used on blank layers.

[photonstorm]

This is fixed in the dev branch. There's no such thing as a null tile now.