Do not require ComboBoxes to be valid if they are not required

Elwig/Windows/MemberAdminWindow.xaml.cs

@@ -1,7 +1,6 @@
 using Microsoft.EntityFrameworkCore;
 using System;
 using System.Collections.Generic;
-using System.ComponentModel;
 using System.Linq;
 using System.Windows;
 using System.Windows.Controls;
@@ -29,7 +28,7 @@ namespace Elwig.Windows {
             };
             RequiredInputs = new Control[] {
                 MgNrInput, GivenNameInput, FamilyNameInput,
-                AddressInput, PlzInput,
+                AddressInput, PlzInput, OrtInput, BillingOrtInput,
                 BusinessSharesInput, BranchInput, DefaultKgInput
             };
         }