Fixed: Null reference error when import list adds new book to existing author

Fixes #1235

(cherry picked from commit 5e3045db86748d0ab407a6785007cb6bbd4f46a6)

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

@@ -105,14 +105,14 @@ namespace NzbDrone.Core.Test.ImportListTests
         {
             Mocker.GetMock<IAuthorService>()
                 .Setup(v => v.FindById(_importListReports.First().AuthorGoodreadsId))
-                .Returns(new Author { ForeignAuthorId = _importListReports.First().AuthorGoodreadsId });
+                .Returns(new Author { Id = 1, ForeignAuthorId = _importListReports.First().AuthorGoodreadsId });
         }
 
         private void WithExistingBook()
         {
             Mocker.GetMock<IBookService>()
                 .Setup(v => v.FindById(_importListReports.First().EditionGoodreadsId))
-                .Returns(new Book { ForeignBookId = _importListReports.First().EditionGoodreadsId });
+                .Returns(new Book { Id = 1, ForeignBookId = _importListReports.First().EditionGoodreadsId });
         }
 
         private void WithExcludedAuthor()
@@ -231,6 +231,7 @@ namespace NzbDrone.Core.Test.ImportListTests
         public void should_add_if_existing_author_but_new_book()
         {
             WithBookId();
+            WithAuthorId();
             WithExistingAuthor();
 
             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;
@@ -262,8 +263,9 @@ namespace NzbDrone.Core.ImportLists
                     }
                 };
 
-                if (importList.ShouldMonitor == ImportListMonitorType.SpecificBook)
+                if (importList.ShouldMonitor == ImportListMonitorType.SpecificBook && toAddAuthor.AddOptions != null)
                 {
+                    Debug.Assert(toAddAuthor.Id == 0, "new author added but ID is not 0");
                     toAddAuthor.AddOptions.BooksToMonitor.Add(toAdd.ForeignBookId);
                 }