Solved the issue with Simon's help
After doing a fresh install, I found a file in the styles folder on my older build called unknown and it contained just a single object and its corresponding txt file. I don't think the editor liked this folder and produced this error. Perhaps because it lacked a theme file?
For reference it was the same trap object you asked me to make for the Marble Garden tileset Nepster, I saved it in the styles folder and forgot about it.
After deleting this, the editor opened again.
Issue resolved but at least there's something to correspond this error message to in the future.