prevent memory leak of mainwindow.

src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs

@@ -122,12 +122,17 @@ namespace Avalonia.Controls.ApplicationLifetimes
                 lifetimeEvents.ShutdownRequested += OnShutdownRequested;
 
             _cts = new CancellationTokenSource();
-            MainWindow?.Show();
+            ShowMainWindow();
             Dispatcher.UIThread.MainLoop(_cts.Token);
             Environment.ExitCode = _exitCode;
             return _exitCode;
         }
 
+        private void ShowMainWindow()
+        {
+            MainWindow?.Show();
+        }
+
         public void Dispose()
         {
             if (_activeLifetime == this)