Fixed: Creating root folders without default tags

src/NzbDrone.Core/RootFolders/RootFolder.cs

@@ -13,7 +13,7 @@ namespace NzbDrone.Core.RootFolders
         public int DefaultQualityProfileId { get; set; }
         public MonitorTypes DefaultMonitorOption { get; set; }
         public NewItemMonitorTypes DefaultNewItemMonitorOption { get; set; }
-        public HashSet<int> DefaultTags { get; set; }
+        public HashSet<int> DefaultTags { get; set; } = new ();
         public bool IsCalibreLibrary { get; set; }
         public CalibreSettings CalibreSettings { get; set; }
 

src/Readarr.Api.V1/RootFolders/RootFolderResource.cs

@@ -108,7 +108,7 @@ namespace Readarr.Api.V1.RootFolders
                 DefaultQualityProfileId = resource.DefaultQualityProfileId,
                 DefaultMonitorOption = resource.DefaultMonitorOption,
                 DefaultNewItemMonitorOption = resource.DefaultNewItemMonitorOption,
-                DefaultTags = resource.DefaultTags,
+                DefaultTags = resource.DefaultTags ?? new HashSet<int>(),
                 IsCalibreLibrary = resource.IsCalibreLibrary,
                 CalibreSettings = cs
             };