How to remove a flat edge from topRight and topLeft in showModalBottomSheet – Flutter

The problem is that your Container has rounded corners.

To fix the problem, add also rounded corners – the same round (30.0) as in the Container to the bottomSheet. You can do so with the shape property:

showModalBottomSheet(
      shape: const RoundedRectangleBorder(
        borderRadius: BorderRadius.vertical(top: Radius.circular(30.0)),
      ),

Complete runnable example:

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MaterialApp(home: MyApp()));
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(body: const Profile());
  }
}

class Profile extends StatefulWidget {
  const Profile({super.key});

  @override
  State<Profile> createState() => _ProfileState();
}

class _ProfileState extends State<Profile> {
  showBottomSheet(BuildContext context) {
    showModalBottomSheet(
      shape: const RoundedRectangleBorder(
        borderRadius: BorderRadius.vertical(top: Radius.circular(30.0)),
      ),
      context: context,
      builder: (context) {
        return Container(
          decoration: BoxDecoration(
            color: Color.fromRGBO(0, 0, 0, 1),
            borderRadius: BorderRadius.only(
              topLeft: Radius.circular(30),
              topRight: Radius.circular(30),
            ),
          ),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            crossAxisAlignment: CrossAxisAlignment.end,
            children: [
              Container(
                height: 36,
              ),
              ListTile(
                leading: Icon(Icons.settings),
                title: Text('Settings'),
              ),
              ListTile(
                leading: Icon(Icons.bookmark_border),
                title: Text('Saved'),
              )
            ],
          ),
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color.fromRGBO(0, 0, 0, 1),
      appBar: AppBar(
        title: Text('test'), // Display the username of user
        backgroundColor: Color.fromRGBO(0, 0, 0, 1),
        actions: [
          IconButton(
            onPressed: () => showBottomSheet(context),
            icon: Icon(Icons.menu),
          ),
        ],
      ),
    );
  }
}