I updated my server to 1.16, only to be greeted with a runtime error regarding letient json.
I reverted back to 1.15.2 and it started up fine.
Later on, I tried 1.16.1, same thing.

After testing a bunch of things, I narrowed it down to the generator-settings string, which contained an old superflat preset string.

Even after updating the string to the latest format in the superflat customization screen, it still throws the error.
It seems it no longer allows this string, and instead tried to find custom world settings instead?

Steps to reproduce:

1. Start a server jar
2. set EULA to true
3. start it again, and see it run fine
4. shut down the server
5. change server.properties to specify a flat level type with generator-settings set to any superflat preset string (for example

   level-type=flat
   generator-settings=minecraft:bedrock,86*minecraft:stone,2*minecraft:dirt,minecraft:mycelium;minecraft:forest

   )

6. start server, and see a letient json runtime error