When i use BoxShadow Colors.grey.withOpacity(0.5) with opacity it show error

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    home: Scaffold(
      appBar: AppBar(
        title: const Text('Welcome'),
      ),
      body: Center(
        child: Container(
          alignment: Alignment.topCenter,
          height: 260,
          width: 260,
          decoration: const BoxDecoration(
            color: Colors.blue,
            //borderRadius: BorderRadius.all(Radius.circular(20)),
            borderRadius: BorderRadius.only(
              topLeft: Radius.circular(20),
              topRight: Radius.circular(30),
              bottomLeft: Radius.elliptical(100, 50),
              bottomRight: Radius.elliptical(70, 150),
            ),
            boxShadow: [
              BoxShadow(
                color: Colors.grey.withOpacity(0.5),
                blurRadius: 10,
                spreadRadius: 5,
                offset: Offset(.5, 0),
                blurStyle: BlurStyle.outer,
              ),
            ],
          ),
          padding: const EdgeInsets.all(10.0),
          margin: const EdgeInsets.only(left: 10),
          child: const Text(
            'Hello World!',
            //textAlign: TextAlign.center,
            style: TextStyle(
              fontSize: 50,
              color: Colors.white,
              fontWeight: FontWeight.bold,
            ),
          ),
        ),
      ),
    ),
  ));
}

Try this, Need to remove const

Center(
        child: Container(
          alignment: Alignment.topCenter,
          height: 260,
          width: 260,
          decoration: BoxDecoration(
              color: Colors.blue,
              borderRadius: BorderRadius.only(
                topLeft: Radius.circular(20),
                topRight: Radius.circular(30),
                bottomLeft: Radius.elliptical(100, 50),
                bottomRight: Radius.elliptical(70, 150),
              ),
              boxShadow: [
                BoxShadow(
                  color: Colors.grey.withOpacity(0.5),
                  blurRadius: 10,
                  spreadRadius: 5,
                  offset: Offset(.5, 0),
                  blurStyle: BlurStyle.outer,
                ),
              ]),
          padding: const EdgeInsets.all(10.0),
          margin: const EdgeInsets.only(left: 10),
          child: const Text(
            'Hello World!',
            //textAlign: TextAlign.center,
            style: TextStyle(
              fontSize: 50,
              color: Colors.white,
              fontWeight: FontWeight.bold,
            ),
          ),
        ),
      ),