After adding the app bar to the app

appbar_main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // I'm building a material app so using the MaterialApp widget gives us access to all of Flutter's material widgets
    return MaterialApp(
      // Implements the basic material design visual layout structure
      home: Scaffold(
        // appBar is a property of the Scaffold widget. This widget creates the top app bar
        appBar: AppBar(
          title: Text(
            'MMR CALCULATOR',
          ),
          // By default, in Android, the the Title is left adjusted but I want it centered. I could either style the Text or use the centerTitle appBar property and set it to true
          centerTitle: true,
        ),
        // Empty body (for now)
        body: null,
      ),
    );
  }
}