Answers for "flutter pop then push"

push and remove until flutter

//named
Navigator.of(context)
    .pushNamedAndRemoveUntil('/login', (Route<dynamic> route) => false);
    
//not named
Navigator.of(context).pushAndRemoveUntil(MaterialPageRoute(builder: (context) =>
    LoginScreen()), (Route<dynamic> route) => false),

flutter push and pop doesnt work

PROBLEM: flutter push and pop doesnt work

SOLUTION:
You are using MaterialApp more then one time in your project, you should only have one instance of MaterialApp 
as it holds your root configuration.

Ctrl + Shift+ F type MaterialApp Search and try to remove MatrialApp with Scaffold
you need to provide only one MaterialApp. And it shoud work...