Comment 6 for bug 1678598

What happens is that grey player has lost their port, so there is no port to return to for the canceled expedition. So, I'd suggest (pseudocode):

if (get_economy() == nullptr) {
  if (has_other_reachable_portdock()) {
    add_ship_to_reachable_portdocks_fleet();
  } else {
    return;
  }
}

do_cancel_expedition;