Customized World Preset Biome ID >= 8 adds 2

To reproduce:

• Create New World.
• Select "Customized" World type.
• Click on "Customize".
• Choose any biome after "River", click "Done".
• Re-click on "Customize", see the slider has moved forward 2 ticks.

Also:

• If "Mesa Plateau F" or "Mesa Plateau" biome is chosen,
• Re-click on "Customize", see the slider is graphically off its track, and the label simply reads "Biome: ?".
• (Does not crash in 1.11.2 and forward, but has previously caused a crash, see attachment)

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Code analysis by Cubitect can be found in this comment.