If you want to reopen your level you can do this:
remove the generated.dat in your Editor\Levels\[LevelName]\AI\ folder
remove the painted.dat in your Editor\Levels\[LevelName]\AI\ folder
remove the surfaces.dat in your Editor\Levels\[LevelName]\AI\ folder
If I remove this 3 files, I could re-open my level. But when I re-generate AI grid it's impossible to reload the level... It crash again, even if I change the size or the paint of my terrain...
Hope it's helpfull...