issue fix

Billing.Shared/Views/CategoryPage.xaml.cs

@@ -146,19 +146,9 @@ namespace Billing.Views
 
         private async void OnCategoryChecked(object sender, Category category)
         {
-            if (category.Id < 0)
+            if (category.Id <= 0)
             {
                 // add
-                int maxId;
-                if (App.Categories.Count > 0)
-                {
-                    maxId = App.Categories.Max(b => b.Id);
-                }
-                else
-                {
-                    maxId = -1;
-                }
-                category.Id = maxId + 1;
                 App.Categories.Add(category);
                 Categories.Add(WrapCategory(category));
             }