Fix GPS tracker initialisation bug

prs.stores/GPSTrackerLocationStore.cs

@@ -28,7 +28,8 @@ namespace Comal.Stores
         {
             _cache = new ConcurrentDictionary<string, Guid>(
                 DbFactory.NewProvider(Logger.Main).Query(
-                    Filter<GPSTracker>.Where(x => x.DeviceID).IsNotEqualTo(Guid.Empty.ToString()),
+                    Filter<GPSTracker>.Where(x => x.DeviceID).IsNotEqualTo(Guid.Empty.ToString())
+                        .And(x => x.DeviceID).IsNotEqualTo(null),
                     Columns.None<GPSTracker>().Add(x => x.DeviceID).Add(x => x.ID)
                 ).Rows
                     .Select(r => new KeyValuePair<string, Guid>(