Not to pick on the devs, being all busy with their new game and all, but I got tired of waiting and did a bit of digging myself. 

So I got the default Tiled map, loaded it up (with Inferno disabled, although I have no idea how to rollback updates), and got pretty much the same error message. 

XML error, no biggie. Did a bit of google-fu, found this used to be an old problem and that the error is simply that the World Generator is running into the following code line: 

 <layer id="4" name="Rivers" width="20" height="20" visible="0">
  <data>
   <tile/>

   <tile/>

... 

... 

Etc. 

This is the 'null' exception. If you open up the Tiled map file in notepad and do replace all <tile/> with this: 

<tile gid="0"/>

You don't get the same error anymore.  You get *2* totally different errors!

Back when EL was early access I actually ran into this error like twice trying to do tiny maps for testing, but I can't remember how I fixed it (I think I just reported it and it got bugfixed). 

The second error is a totally different monster.

I did a search in my map file, and the only 'index' parameter I could find was this (underlined)

<?xml version="1.0" encoding="UTF-8"?>
<map version="1.2" tiledversion="1.2.2" orientation="hexagonal" renderorder="left-down" width="20" height="20" tilewidth="60" tileheight="52" infinite="0" hexsidelength="26" staggeraxis="y" staggerindex="even" nextlayerid="7" nextobjectid="1">
 <tileset firstgid="1" name="Elevations" tilewidth="60" tileheight="80" tilecount="12" columns="4">

That's as far as my coding knowledge goes, and I've got no idea where I'd even start looking from here. I'd contact the Tiled guy, get his advice on it, but I don't think he was ever involved in Endless Legend's coding. 

Any advice on what I can do to find out more about this from the devs or someone smarter with code would be appreciated.