When creating a new superflat world with the overworld preset (regardless of gamemode or difficulty) the game crashes with java.lang.NullPointerException: mouseClicked event handler.  The error results in a world save folder being created in the .minecraft folder, but said folders are empty. 

What I expected to happen was...:

I expected a superflat world with the overworld preset to be generated to play on.

What actually happened was...:

What happened was that the game appeared to load the world, only to immediately crash to the launcher with the following error message "The game crashed whilst mouseclicked event handler
Error: java.lang.NullPointerException: mouseClicked event handler"

The world folder shows up in .minecraft folder, but are empty of contents.

Steps to Reproduce:
1. Load Minecraft 20w21a and create new world.
3. Select 'more world options' and select the world type to be superflat.
4. Customize the superflat world by selecting the overworld preset. Do not change anything about the preset.
5. Select done and create the world.