Unsupported operation: Cannot add to an unmodifiable list

I have an app that has a splash screen and an onboarding screen. There are no errors or warnings anywhere; the app runs to show the splash screen but then crashes instead of displaying the onboarding screen.

======== Exception caught by widgets library =======================================================
The following UnsupportedError was thrown building BoardingPage(dirty, state: _BoardingScreenState#e3368):
Unsupported operation: Cannot add to an unmodifiable list

The relevant error-causing widget was: 
  BoardingPage BoardingPage:file:///C:/Users/Srishti/AndroidStudioProjects/App-mini-project-1/lib/splash.dart:22:78

Here's my code :

splash.dart

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

class Splash extends StatefulWidget {
  const Splash({Key? key}) : super(key: key);
  @override
  State<Splash> createState() => _SplashState();
}

class _SplashState extends State<Splash> {
  @override
  void initState(){
    super.initState();
    _navigatetohome();
  }
  
  _navigatetohome() async {
    await Future.delayed(Duration(milliseconds: 2500), (){});
    Navigator.pushReplacement(context, MaterialPageRoute(builder: (context) => BoardingPage()));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Container(
          child: Text('Your Scheduler', 
            style: TextStyle(               
              fontSize: 32,
              fontWeight: FontWeight.normal,
            ),
          ),
        ),
      ),
    );
  }
}

slide.dart

class Slide {
  String image;
  String heading;

  Slide(this.image, this.heading);
}

boarding_screen.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:gradient_widgets/gradient_widgets.dart';
import 'package:schedule_management/slide.dart';
import 'login_screen.dart';

class BoardingPage extends StatefulWidget {
  const BoardingPage({Key? key}) : super(key: key);
  @override
  _BoardingScreenState createState() => _BoardingScreenState();
}

class _BoardingScreenState extends State<BoardingPage> {

  int _currentPage = 0;
  List<Slide> _slides = [];
  PageController _pageController = PageController();

  @override
  void initState() {
    _currentPage = 0;
    _slides = [
      Slide("images/slide-1.png", "Manage your time"),
      Slide("images/slide-2.png", "Schedule your tasks"),
      Slide("images/slide-3.png", "Never miss out on any task"),
    ];
    _pageController = PageController(initialPage: _currentPage);
    super.initState();
  }

  // the list which contain the build slides
  List<Widget> _buildSlides() {
    return _slides.map(_buildSlide).toList();
  }

  // building single slide

  Widget _buildSlide(Slide slide) {
    return Column(
      children: <Widget>[
        Expanded(
          child: Container(
            margin: const EdgeInsets.all(1),
            child: Image.asset(slide.image, fit: BoxFit.contain),
          ),
        ),
        Padding(
          padding: const EdgeInsets.symmetric(horizontal: 70),
          child: Text(
            slide.heading,
            textAlign: TextAlign.center,
            style: const TextStyle(
              fontSize: 28,
              fontWeight: FontWeight.w900,
            ),
          ),
        ),
        const SizedBox(
          height: 230,
        )
      ],
    );
  }

  // handling the on page changed
  void _handlingOnPageChanged(int page) {
    setState(() => _currentPage = page);
  }

  // building page indicator
  Widget _buildPageIndicator() {
    Row row = Row(mainAxisAlignment: MainAxisAlignment.center, children: const []);
    for (int i = 0; i < _slides.length; i++) {
      row.children.add(_buildPageIndicatorItem(i));
      if (i != _slides.length - 1) {
        row.children.add(const SizedBox(
          width: 12,
        ));
      }
    }
    return row;
  }

  Widget _buildPageIndicatorItem(int index) {
    return Container(
      width: index == _currentPage ? 8 : 5,
      height: index == _currentPage ? 8 : 5,
      decoration: BoxDecoration(
          shape: BoxShape.circle,
          color: index == _currentPage
              ? const Color.fromRGBO(136, 144, 178, 1)
              : const Color.fromRGBO(206, 209, 223, 1)),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      body: Stack(
        children: <Widget>[
          PageView(
            controller: _pageController,
            onPageChanged: _handlingOnPageChanged,
            physics: BouncingScrollPhysics(),
            children: _buildSlides(),
          ),
          Positioned(
            left: 0,
            right: 0,
            bottom: 0,
            child: Column(
              children: <Widget>[
                _buildPageIndicator(),
                SizedBox(height: 32,),
                Container(
                  // see the page indicators
                  margin: EdgeInsets.symmetric(horizontal: 10000000),
                  child: SizedBox(
                      width: double.infinity,
                      child: GradientButton(
                        callback: () => {},
                        gradient: LinearGradient(colors: const [
                          Color.fromRGBO(11, 198, 200, 1),
                          Color.fromRGBO(68, 183, 183, 1)
                        ]),
                        elevation: 0,
                        increaseHeightBy: 28,
                        increaseWidthBy: double.infinity,
                        shape: RoundedRectangleBorder(
                          borderRadius: BorderRadius.circular(100),
                        ),
                        child: Text(
                          "",
                          style: TextStyle(
                            letterSpacing: 4,
                            fontSize: 16,
                            fontWeight: FontWeight.bold,
                            color: Colors.white,
                          ),
                        ),
                      )),
                ),
                SizedBox(height: 10,),
                CupertinoButton(
                    child: Text(
                      "Sign In",
                      style: TextStyle(
                        fontSize: 20,
                        fontWeight: FontWeight.w700,
                        color: Colors.grey,
                      ),
                    ),
                    onPressed: () {
                      Navigator.push(context, MaterialPageRoute(builder: (context) => LoginScreen()));
                    }),
                SizedBox(height: 30,),
              ],
            ),
          )
        ],
      ),
    );
  }
}

I have tried restarting Android Studio and running flutter clean but the app still crashes.