Fix when closing MainWindow to set preferred language to null.

Author: giladreich

src/Moodle.WebCrawler/Views/MainWindow.cs

@@ -188,6 +188,7 @@ private void MainWindow_FormClosing(object sender, FormClosingEventArgs e)
             {
                 e.Cancel = true;
                 ResetUserLanguageToDefault();
+                userPreferredLanguage = null;
 
                 Close();
             }