AdministrationWindow: Fix default value handling

Elwig/Helpers/ControlUtils.cs

@@ -203,5 +203,21 @@ namespace Elwig.Helpers {
         public static void SelectCheckComboBoxItems(Xceed.Wpf.Toolkit.CheckComboBox ccb, IEnumerable<object>? items, Func<object?, object?> getId) {
             SelectCheckComboBoxItems(ccb, getId, items?.Select(i => getId(i)));
         }
+
+        public static object? GetInputValue(Control input) {
+            if (input is TextBox tb) {
+                return tb.Text;
+            } else if (input is ComboBox sb) {
+                return sb.SelectedItem;
+            } else if (input is Xceed.Wpf.Toolkit.CheckComboBox ccb) {
+                return ccb.SelectedItems.Cast<object>().ToArray();
+            } else if (input is CheckBox cb) {
+                return cb.IsChecked;
+            } else if (input is RadioButton rb) {
+                return rb.IsChecked;
+            } else {
+                return null;
+            }
+        }
     }
 }