QueryWindow: Minor improvements

Elwig/Windows/MailWindow.xaml.cs

@@ -90,7 +90,7 @@ namespace Elwig.Windows {
             private set => SetValue(EmailWishCountProperty, value);
         }
 
-        private ICommand _deleteCommand;
+        private ICommand? _deleteCommand;
         public ICommand DeleteCommand => _deleteCommand ??= new ActionCommand(() => {
             var idx = SelectedDocumentsList.SelectedIndex;
             if (idx == -1)