The following error gets thrown in sandbox mode:
Error while running event far-reach::on_player_joined_game (ID 43)
No character.
stack traceback:
far-reach/control.lua:5: in function 'far_reach_apply_settings'
far-reach/control.lua:24: in function <far-reach/control.lua:23>
Because of this, going between a freeplay and sanbox save requires disabling the mod and restarting factorio. :(