Ensure TravelHistoryControl.visitedSystems is never null
Ensure TravelHistoryControl.visitedSystems is never null

EDDiscovery/EDDiscovery,EDDiscovery/EDDiscovery,klightspeed/EDDiscovery,andreaspada/EDDiscovery,andreaspada/EDDiscovery,EDDiscovery/EDDiscovery,klightspeed/EDDiscovery,klightspeed/EDDiscovery