The mod Biter Cleanup (3.0.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event biter-cleanup::on_nth_tick(1)
biter-cleanup/control.lua:133: attempt to index local 'surface' (a nil value)
stack traceback:
biter-cleanup/control.lua:133: in function 'delete_random_decal'
biter-cleanup/control.lua:232: in function <biter-cleanup/control.lua:225>

Thanks for the report, I'll be able to push a fix for this in a couple of days - for now here's a workaround:
- Save the game just before deleting the surface
- Remove or disable the biter cleanup mod, then load the save without syncing mods
- Delete the surface, save again
- Re-add the mod if you feel so inclined

It should create a fresh list of biter decals that doesn't include any references to the new surface and allow you to carry on with your save

https://pastebin.com/JaXWaL32

This is my proposed fix. It worked fine for me at least.

Thanks for the proposed fix, it does look good - only reason I didn't want to implement it was because the entire algorithm used by the mod at that time was... not my favourite. The new version is out though, and it should impact save times a lot less, and also just not have this issue.