Can't add leader fallback in asset editor[solved]

[DeepSoul]

I'm trying to modify a custom civ with my own art. I can add the userinterface leader portrait, but when I try to add the fallback image a messagebox about exporting keeps flashing forever. What's wrong and what's the proper way of adding leader fallbacks?

Edit: After emptying the target directory I managed to add it. It doesn't show a preview in asset editor but visible in the game.