async Task ExecuteLoadItemsCommand()
        {
            if (IsBusy)
            {
                return;
            }

            IsBusy = true;

            try
            {
                Items.Clear();
                ItemsGrouped.Clear();
                var items = await DataStoreNew.GetItemsAsync <Invoice>();

                foreach (var item in items)
                {
                    Items.Add(item);
                }


                foreach (var group in items.GroupBy(x => x.SellerName))
                {
                    ItemsGrouped.Add(group);
                }
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex);
            }
            finally
            {
                IsBusy = false;
            }
        }
        public InvoiceListViewModel()
        {
            Title                    = "Invoices";
            Items                    = new ObservableCollection <Invoice>();
            ItemsGrouped             = new ObservableCollection <IGrouping <string, Invoice> >();
            LoadItemsCommand         = new Command(async() => await ExecuteLoadItemsCommand());
            ReloadInvoicesFromServer = new Command(async() => await ExecuteReloadInvoicesFromServerCommand());

            MessagingCenter.Subscribe <NewItemPage, Invoice>(this, "AddItem", async(obj, item) =>
            {
                var _item = item as Invoice;
                Items.Add(_item);
                await DataStoreNew.AddItemAsync(_item);
            });
        }