Fix NullReferenceException

src/Tgstation.Server.Host/Components/Watchdog/WatchdogBase.cs

@@ -434,6 +434,11 @@ namespace Tgstation.Server.Host.Components.Watchdog
 				var notification = new EventNotification(eventType, parameters);
 
 				var activeServer = GetActiveController();
+				
+				// Server may have ended
+				if (activeServer == null)
+					return true;
+
 				result = await activeServer.SendCommand(
 					new TopicParameters(notification),
 					cancellationToken)