I can’t fetch the data from the post server, I get a fatal error

I have a link to a post server; I am trying to build a mini app to fetch and display data from this link in the App. I created network request to fetch the HTML

import 'package:http/http.dart' as http;

class NetworkService {
  final String url;

  NetworkService(this.url);

  Future<String> fetchHtml() async {
    final response = await http.get(Uri.parse(url));
    if (response.statusCode == 200) {
      return response.body;
    } else {
      throw Exception('Failed to load HTML');
    }
  }
}

And display in WebView

`import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'package:graph_display_app/services/network_service.dart';

class GraphScreen extends StatefulWidget {
  final String url;

  const GraphScreen({Key? key, required this.url}) : super(key: key);

  @override
  _GraphScreenState createState() => _GraphScreenState();
}

class _GraphScreenState extends State<GraphScreen> {
  late WebViewController controller;
  String? htmlContent;
  bool isLoading = true;

  @override
  void initState() {
    super.initState();
    fetchGraph();
  }

  Future<void> fetchGraph() async {
    try {
      NetworkService networkService = NetworkService(widget.url);
      String html = await networkService.fetchHtml();
      setState(() {
        htmlContent = html;
        isLoading = false;
        controller = WebViewController()
          ..loadRequest(
            Uri.dataFromString(
              htmlContent!,
              mimeType: 'text/html',
              encoding: Encoding.getByName('utf-8'),
            ),
          );
      });
    } catch (e) {
      setState(() {
        isLoading = false;
      });
      ScaffoldMessenger.of(context).showSnackBar(
        const SnackBar(content: Text('Failed to load graph')),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Graph Viewer'),
      ),
      body: isLoading
          ? const Center(child: CircularProgressIndicator())
          : WebViewWidget(controller: controller),
    );
  }
}

I get a fatal error when I debug. Could it be the I have implemented wrongly?