Why no error with static varibale?

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    ScreenUtil.init(context);
    print(Themes.xs);
    return Container();
  }
}


class ScreenUtil {
  ScreenUtil._();
  
  static ScreenUtil _instance;
  
  factory ScreenUtil() {
    return _instance;
  }
  
  static void init(BuildContext context) {
    if (_instance == null) {
      _instance = ScreenUtil._();
    }
  }
  
  num setSp(num value) => value;
}

extension ScreenUtilExtension on num {
  num get sp => ScreenUtil().setSp(this);
}


class Themes {
  static final xs = 10.sp;
}