这是因为popupMenuItem的onTap试图使用Navigator.pop来关闭弹出窗口，但同时您试图显示对话框，因此它关闭对话框并离开弹出窗口，因此，您可以等到所有动画或正在进行的事情完成后再显示对话框
代码：dartPad code

PopupMenuItem(
              child: const Text('Item 0'),
              onTap: () {
                WidgetsBinding?.instance?.addPostFrameCallback((_) {
                  showCupertinoDialog(
                      context: context,
                      builder: (context) {
                        return CupertinoAlertDialog(
                          title: const Icon(CupertinoIcons.info_circle),
                          content: const Text(
                            'Hello User, Welcome',
                            textAlign: TextAlign.center,
                          ),
                          actions: [
                            CupertinoDialogAction(
                              isDefaultAction: true,
                              onPressed: () => Navigator.pop(context),
                              child: const Text('Thanks'),
                            ),
                          ],
                        );
                      });
                });
              }),

试试这个：

PopupMenuItem(
  child: Text("Show dialog"),
  onTap: () { 
      Future<void>.delayed(
        Duration.zero,
        () => showDialog(
              context: context,
              barrierColor: Colors.black26,
              builder: (context) => AlertDialog(...),
        ),
    ),
  },
);