How to use onTap or onPressed in PopupMenuItem

There is a property called onSelected, you should use it, it handles onTap event.

PopupMenuButton(
  icon: Icon(Icons.settings),
  onSelected: (newValue) { // add this property
    setState(() {
      _value = newValue; // it gives the value which is selected
    });
  },
  itemBuilder: (context) => [
    PopupMenuItem(
      child: Text("Settings"),
      value: 0,
    ),
    PopupMenuItem(
      child: Text("Flutter.io"),
      value: 1,
    ),
    PopupMenuItem(
      child: Text("Google.com"),
      value: 2,
    ),
  ],
)

Just add this to your PopupMenuButton:

onSelected: (result) {
    if (result == 0) {
        Navigator.push(
            context,
            MaterialPageRoute(builder: (context) => SettingPage()),
        );
    }
},

And change your setting button to:

PopupMenuItem(
    child: Text("Settings"),
    value: 0,
),

I faced similar issues while navigating the screen using pop up menu button and I solve the issues by putting this method inside the onTap callback of PopupMenuItem:

    onTap: (){
WidgetsBinding.instance!.addPostFrameCallback((_) {
                      Navigator.push(
                        context,
                        MaterialPageRoute(
                          builder: (context) {
                            return ScreenName();
                          },
                        ),
                      );
                    });
}