destroy_overlay (scripts/overlay.lua around lines 236-241) calls self.background.destroy() and self.mapbackground.destroy() without checking valid. The background can already be gone: iterator.clear_all and clear_hovered call renderer.reset(), which clears all of the mod's render objects when no player has an iterator left. Toggling the overlay after that should stop the game (found by reading the code, not reproduced). if self.background and self.background.valid then self.background.destroy() end, and the same for mapbackground, avoids it.

The overlay also stays on the surface where it was created: after a trip on a platform or to another planet only its target changes, so the new surface has no darkening until the overlay is toggled. Recreating the background in update_overlay when self.background.surface ~= self.player.surface fixes that.