Fixed: Null reference error when import list adds new album to existing artist

(cherry picked from commit 5e3045db86748d0ab407a6785007cb6bbd4f46a6)

src/NzbDrone.Core.Test/ImportListTests/ImportListSyncServiceFixture.cs

@@ -89,14 +89,14 @@ namespace NzbDrone.Core.Test.ImportListTests
         {
             Mocker.GetMock<IArtistService>()
                 .Setup(v => v.FindById(_importListReports.First().ArtistMusicBrainzId))
-                .Returns(new Artist { ForeignArtistId = _importListReports.First().ArtistMusicBrainzId });
+                .Returns(new Artist { Id = 1, ForeignArtistId = _importListReports.First().ArtistMusicBrainzId });
         }
 
         private void WithExistingAlbum()
         {
             Mocker.GetMock<IAlbumService>()
                 .Setup(v => v.FindById(_importListReports.First().AlbumMusicBrainzId))
-                .Returns(new Album { ForeignAlbumId = _importListReports.First().AlbumMusicBrainzId });
+                .Returns(new Album { Id = 1, ForeignAlbumId = _importListReports.First().AlbumMusicBrainzId });
         }
 
         private void WithExcludedArtist()
@@ -215,6 +215,7 @@ namespace NzbDrone.Core.Test.ImportListTests
         public void should_add_if_existing_artist_but_new_album()
         {
             WithAlbumId();
+            WithArtistId();
             WithExistingArtist();
 
             Subject.Execute(new ImportListSyncCommand());

src/NzbDrone.Core/ImportLists/ImportListSyncService.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using NLog;
 using NzbDrone.Common.Extensions;
@@ -243,8 +244,9 @@ namespace NzbDrone.Core.ImportLists
                     }
                 };
 
-                if (importList.ShouldMonitor == ImportListMonitorType.SpecificAlbum)
+                if (importList.ShouldMonitor == ImportListMonitorType.SpecificAlbum && toAddArtist.AddOptions != null)
                 {
+                    Debug.Assert(toAddArtist.Id == 0, "new artist added but ID is not 0");
                     toAddArtist.AddOptions.AlbumsToMonitor.Add(toAdd.ForeignAlbumId);
                 }