main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  void _getCsvDocunent(BuildContext ctx) async {
    if (ctx != null) _showAlert(ctx, 'Hello world');
  }

  Future _showAlert(BuildContext context, String message) async {
    return showDialog(
        context: context,
        child: new AlertDialog(
          title: new Text(message),
          actions: <Widget>[
            new FlatButton(
                onPressed: () => Navigator.pop(context), child: new Text('Ok'))
          ],
        ));
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Plugin example app'),
        ),
        body: Center(
          child: Column(
            children: <Widget>[
              FlatButton(
                child: Text('Show alert'),
                onPressed: () => _getCsvDocunent(
                    context), // показать алерт из _getCsvDocunent
              ),
            ],
          ),
        ),
      ),
    );
  }
}