Hey Casper! I was also dealing with the error that some others had:
TypeError
Cannot read property 'tilesetNames' of null
I found the cause to be when I have an event on the title map that has a piece of the tileset selected as the image, so from [Tileset A/B/C]. Removing the event fixes the problem.
I just worked around it by doing some different layering, but just FYI in case other people have this issue.