optimized and add diagnostic feature

Billing.Shared/Models/Category.cs

@@ -8,6 +8,9 @@ namespace Billing.Models
         private const string ICON_DEFAULT = "ic_default";
         private const long TRANSPARENT_COLOR = 0x00ffffffL;
 
+        private static Category empty;
+        public static Category Empty => empty ??= new() { Id = -1 };
+
         [PrimaryKey, AutoIncrement]
         public int Id { get; set; }
         public CategoryType Type { get; set; }