Icons used for customizing superflat worlds are not displayed if no world has been opened before

The bug

On the superflat customization screen, the layer icons and the preset icons aren't displayed if no world has been played in since the game has been launched.

To reproduce

1. Close Minecraft, if it is open
2. Launch Minecraft
3. Enter the "Create new World" screen
4. Select "Superflat" as world type
5. Click "Customize"
   → The layer icons don't display
6. Click "Presets"
   → The preset icons don't display