[0.4.1/Mac] Crash when generating map with all terrains None

[nessumo]

Steps to reproduce

• Play > New game
• Select Dirt/Grass/Hills/Sand to None
• Generate

Observed

• Freeze. No response from buttons, keyboard shortcuts etc. Error sound on every click. No way to close the game (I was on fullscreen).

[kovarex]

This is fixed for 0.6.3 (as well as better messages when non valid numbers are written in the width/height fields).