Maybe something went wrong while you were making a level and the configuration file got messed up. Does it do that for every level, or just one? If the former, try reinstalling the Manager, if the latter, the level properties for the level might be corrupted, and you should try editing World.ini manually. Finally, if this only happens once in a while, I have no idea what to do.
Does the same error happen if you select (current screen) instead?