The bug

Creating a Superflat "The Void" world will not generate a starting platform after changing the biome.

Steps to reproduce

1. World type select Superflat.
2. Click the Customize button.
3. Click the Presets button.
4. Click on the Preset "The Void".
5. Changing the biome from "the_void" to "savanna".
6. Create the world.
7. Found that the starting platform does not generate.

Please note that this is not consistent with other Presets world,
if you do the same for the Preset "Overworld" it will still generate the structure.