replace offstage with if-else for LinearProgressIndicator in dialog

Signed-off-by: 21pages <pages21@163.com>

flutter/lib/desktop/pages/desktop_setting_page.dart

@@ -2065,9 +2065,9 @@ void changeSocks5Proxy() async {
                 ),
               ],
             ),
-            Offstage(
-                offstage: !isInProgress,
-                child: const LinearProgressIndicator().marginOnly(top: 8))
+            // NOT use Offstage to wrap LinearProgressIndicator
+            if (isInProgress)
+              const LinearProgressIndicator().marginOnly(top: 8),
           ],
         ),
       ),