An alert pops up after you open a savegame:
Error while running event Gun_Turret_Alerts::on_tick (ID 0) Gun_Turret_Alerts/control.lua:9: attempt to index field 'turret_entities' (a nil value)
stack traceback: Gun_Turret_Alerts/control.lua:9: in function <Gun_Turret_Alerts/control.lua:4>