Setting level-type to minecraft:flat in server.properties without changing the value of generator-settings from {} generates the following error in the console when the world starts to generate:

[timestamp] [ServerMain/ERROR]: No key layers in MapLike[{}]

How to reproduce:

1. In a dedicated server environment, open the server.properties file.
2. Set level-type to minecraft\:flat. (The backslash is so that the java Properties format is satisfied.)
3. Without changing the value of generator-settings from the default, save the file.
4. Start the server and look at the console.

Expected result:

The server would see the empty object as the value of generator-settings and ignore it, generating the world without errors.

Observed result:

The server outputs an error because of the empty generator-settings object.