The argument type 'Function' can't be assigned to the parameter type 'void Function()?' after null safety

Change your code to accept a VoidCallback instead of Function for the onPressed.Btw VoidCallback is just shorthand for void Function() so you could also define it as final void Function() onPressed;

Updated code:

class DrawerItem extends StatelessWidget {          final String text;      final VoidCallback onPressed;          const DrawerItem({Key key, this.text, this.onPressed}) : super(key: key);          @override      Widget build(BuildContext context) {        return FlatButton(          child: Text(            text,            style: TextStyle(              fontWeight: FontWeight.w600,              fontSize: 18.0,            ),          ),          onPressed: onPressed,        );      }    }

With null safety:

Instead of

final Function? onPressed;

use

final VoidCallback? onPressed;

or

final Function()? onPressed;

Well that's because onPressed inside FlatButton is not a normal function its VoidCallBack Function.You can try something like this:

final VoidCallBack onPressed;

While, you are passing a normal function into a VoidCallBack

Follow the official doc here

Updated Code:

import 'package:flutter/material.dart';void main() => runApp(new MyApp());class MyApp extends StatelessWidget {  @override  Widget build(BuildContext context) {    return MaterialApp(      home: HomeScreen(),    );  }}class HomeScreen extends StatelessWidget {  _myFunction() => print("Being pressed!");  @override  Widget build(BuildContext context) {    return Scaffold(      body: Center(        child: Column(          mainAxisAlignment: MainAxisAlignment.spaceEvenly,          children: [            DrawerItem(              text: "Hello Jee",              onPressed: _myFunction,            ),          ],        ),      ),    );  }}class DrawerItem extends StatelessWidget {  final String text;  final Function onPressed;  const DrawerItem({Key key, this.text, this.onPressed}) : super(key: key);  @override  Widget build(BuildContext context) {    return FlatButton(      child: Text(        text,        style: TextStyle(          fontWeight: FontWeight.w600,          fontSize: 18.0,        ),      ),      onPressed: onPressed,    );  }}