Getting "NoSuchMethodError: The method 'toImage' was called on null." Exception about screenshot HOT 6 CLOSED

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  bool _isSaving = false;
  List<ScreenshotController> _screenshotControllers = [];
  List<Widget> _widgets = [
    Card(
      child: ListTile(
        leading: Icon(
          Icons.person,
        ),
        title: Text('Testing Screenshot 1'),
      ),
    ),
    Card(
      child: ListTile(
        leading: Icon(
          Icons.person,
        ),
        title: Text('Testing Screenshot 2'),
      ),
    ),
  ];

  @override
  Widget build(BuildContext context) {
    _screenshotControllers.clear();

    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: SingleChildScrollView(
        child: Column(
          children: _widgets.map((e) {
            ScreenshotController controller = ScreenshotController();
            _screenshotControllers.add(controller);
            return Screenshot(
              controller: controller,
              child: e,
            );
          }).toList(),
        ),
      ),
      floatingActionButton: _isSaving
          ? CircularProgressIndicator(
              valueColor: AlwaysStoppedAnimation(Colors.white),
            )
          : FloatingActionButton(
              onPressed: _saveWidgets,
              tooltip: 'Save',
              child: Icon(Icons.save),
            ),
    );
  }

  void _saveWidgets() async {
    setState(() {
      _isSaving = true;
    });
    try {
      for (int i = _screenshotControllers.length - 1; i >= 0; --i) {
        var data =
            await _screenshotControllers[i].capture(pixelRatio: 3);
        print('ScreenshotData: $data');
      }
    } catch (e) {
      print('failed to capture: $e');
      debugPrintStack();
    }
    setState(() {
      _isSaving = false;
    });
  }
}