DeliveryAdminWindow: Use last scale error as default manual weighing reason

Elwig/Windows/AdministrationWindow.cs

@@ -413,7 +413,7 @@ namespace Elwig.Windows {
             UpdateButtons();
         }
 
-        protected void TextBox_TextChanged(object sender, RoutedEventArgs evt) {
+        protected void TextBox_TextChanged(object sender, RoutedEventArgs? evt) {
             var input = (TextBox)sender;
             if (SenderIsRequired(input) && input.Text.Length == 0) {
                 ValidateInput(input, false);