Prevent NullRef when deleting missing backups

(cherry picked from commit 0ff0fe2e68f3abf7b8e4d6bf0c1e9dee4eb68227)

src/Sonarr.Api.V3/System/Backup/BackupController.cs

@@ -20,7 +20,7 @@ namespace Sonarr.Api.V3.System.Backup
         private readonly IAppFolderInfo _appFolderInfo;
         private readonly IDiskProvider _diskProvider;
 
-        private static readonly List<string> ValidExtensions = new List<string> { ".zip", ".db", ".xml" };
+        private static readonly List<string> ValidExtensions = new () { ".zip", ".db", ".xml" };
 
         public BackupController(IBackupService backupService,
                             IAppFolderInfo appFolderInfo,
@@ -53,6 +53,12 @@ namespace Sonarr.Api.V3.System.Backup
         public void DeleteBackup(int id)
         {
             var backup = GetBackup(id);
+
+            if (backup == null)
+            {
+                throw new NotFoundException();
+            }
+
             var path = GetBackupPath(backup);
 
             if (!_diskProvider.FileExists(path))