Since updating the Mod to Version 2.5.1 today I get an error message every time I want to create a new stage and get thrown in the Factorio Main Menu. Yesterday before I did the update it worked fine. Screenshot of the error message:

https://postimg.cc/jCcDMTY8

Thanks for reporting!

I couldn't reproduce the bug, but I had a good guess what the issue could be.
Try the latest patch (2.5.2) and let me know if it's fixed!

Tried it and I got the same error Message. I noticed that the new stage got created right before the error occurred, at least the newly created stage 2.11 appears in the list on the left, see new screenshot below.

https://postimg.cc/zLRdry2z

So, I had some time to try a few things. Reverting to earlier versions of the mod and several other things i tried (like reinstalling the game) didn't resolve the problem. At least not in my existing projects. But in newly created projects it works now, i can create new stages again.
No idea how and why my old projects seem to be gotten corrupted, on the day the bug occurred I had worked on my project for a few hours (and had been creating new stages), then quit the game to run errands, restarted a few hours later, updated the mod (and only this one mod) and the bug happened. Then my logical conclusion considering this and the error message was a bug with the mod. My conclusion now is that propably it has nothing to do with the mod and instead is a problem with my savefiles. Maybe that will change again if it happens again...
We'll see, for now I can work with what I have.

I am getting the same error whenever i try to add a new stage on the current mod with an existing file. In my testing, it has something to do with an existing save file. If i make a new file, then paste in my exisiting BPs to recreate, it seems to work fine.
I really don't want to recreate all my files, but this seems the only way. If you would like a save file to test this out, let me know.

I think what happened, is the mod was loaded in a earlier version where a migration wasn't done properly, then saved again. It's hard to tell exactly what went wrong...

For migrating the project to a new save, if you can successfully load the mod in one of the later versions, it should be possible to export the entire project to a string and import in another project.

If that still doesn't work; please share a save, and I'll see what I can recover from it.
Maybe this can be automatically recovered in future mod releases.