Minecraft wiki entry for Development_versions, associated text: MC-194933 – Game crashes when loading a custom biome: java.lang.UnsupportedOperationException: Trying to add tag of type 8 to list of 10.
MC-197197 – Game crashes or freezes after creating or entering a Superflat world without presets.
MC-197225 – Crash: java.util.concurrent.CompletionException: u: Exception generating new chunk.