public async Task AddOrUpdateTargetAsync(IBackupSyncTarget target)
        {
            _targets.AddOrUpdate(target);

            var b = _targets.Items;
            await _cache.InsertAsync("settings_backupSync_targets", b, _serializer).ConfigureAwait(false);
        }
        public async Task RemoveTargetAsync(IBackupSyncTarget target)
        {
            Guard.IsNotNull(target, nameof(target));

            _targets.Remove(target);

            var a = _targets.Items;

            if (!a.Any())
            {
                await _cache.RemoveAsync("settings_backupSync_targets");
            }
            else
            {
                await _cache.InsertAsync("settings_backupSync_targets", a, _serializer).ConfigureAwait(false);
            }
        }