ControlUtils: Cleanup SelectItem() method and use accordingly

Elwig/Helpers/Billing/Varibute.cs

@@ -1,4 +1,5 @@
 using Elwig.Models.Entities;
+using Microsoft.EntityFrameworkCore;
 using System;
 using System.Collections.Generic;
 
@@ -45,6 +46,7 @@ namespace Elwig.Helpers.Billing {
         }
     }
 
+    [PrimaryKey("Listing")]
     public class Varibute : IComparable<Varibute> {
 
         public WineVar? Variety { get; }