Why the type of `value` is `int?` and not `int`? in what case it would be `null`?

[stephane-archer]

import 'package:async_builder/async_builder.dart';
import 'package:flutter/material.dart';

class ClassName extends StatelessWidget {
  final Future<int> _f;
  const ClassName(Future<int> f, {super.key}) : _f = f;

  @override
  Widget build(BuildContext context) {
    return AsyncBuilder<int>(
      future: _f,
      waiting: (context) => Text('waiting'),
      error: (context, error, stackTrace) => Text('error $error'),
      builder: (BuildContext context, int? value) {
        return Text('the value is $value');
      },
    );
  }
}

Why the type of value is int? and not int? in what case it would be null?