Its fixable for worlds corrupted. Open your level.dat with NBTEditor, then navigate to [Your World's Name] > level.dat > DragonFight. There should be a list of numbers from 0 to 19, randomly ordered, but the list will be empty. You can just add the numbers from 0 to 19, or create a new world and open this new world with NBTEdit, then copy the list from the new world to the old one.
Its fixable for worlds corrupted. Open your level.dat with NBTEditor, then navigate to [Your World's Name] > level.dat > DragonFight. There should be a list of numbers from 0 to 19, randomly ordered, but the list will be empty. You can just add the numbers from 0 to 19, or create a new world and open this new world with NBTEdit, then copy the list from the new world to the old one.