This particular exception is probably because
NewMap is invalid or doesn't exist.
There are a whole bunch of reasons why this might happen.
- The first thing I would suggest is checking to see if the TMX file is building with the Content Pipeline tool
- Make sure you've got the name correct including the casing (e.g. "newmap" is not the same as "NewMap")
- Try loading a different map file, maybe one out of the demos.
- Make sure you've also got all of the maps dependencies in the Content Pipeline (e.g. textures, etc).
If you're still having trouble I suggest raising an issue on github and explain what you've tried and attach a copy of the map that you're trying to load (or another one that has the problem).