I've modded my ImprovementDefs.xml but I guess I'm missing something else, because I have weird placeholders in my game, like these:
Can anyone help me please?
EDIT: I just deleted the "<IsIndestructible>true</IsIndestructible>" trait from most of the improvements, except colony capitals (for historical reasons). Seriously, you can move billions of people between stars, but you can't relocate a building or a city on a planet?