Future<HttpClient> is not a subtype of HttpClient

[Durgeshsinghn70]

import 'dart:io';    
import 'package:dio/dio.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:flutter_app_retrofit/example.dart';

class Homepage extends StatefulWidget {
@override
HomepageState createState() => HomepageState();
}

class _HomepageState extends State<Homepage> {
@override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.white,
body: Center(
child: FutureBuilder(
future: this._getPosts(),
builder: (context, snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.none:
return CircularProgressIndicator();
break;
case ConnectionState.waiting:
return CircularProgressIndicator();
break;
case ConnectionState.active:
return CircularProgressIndicator();
break;
case ConnectionState.done:
return CircularProgressIndicator();
break;

default:
return CircularProgressIndicator();
     }
   },),
 ),
);
}

Future _getPosts() async {
final dio= Dio();
(dio.httpClientAdapter as DefaultHttpClientAdapter).onHttpClientCreate = await (client) async {
SecurityContext securityContext=new SecurityContext();
final ByteData crtData = await rootBundle.load('assets/server.crt');
securityContext.setTrustedCertificatesBytes(crtData.buffer.asUint8List());
final ByteData keyBytes = await rootBundle.load('assets/server.key');
securityContext.usePrivateKeyBytes(keyBytes.buffer.asUint8List());
return HttpClient(context: securityContext);
};
final client = RestClient(dio);
client.login('madhusudhan@****.in', '88888');

}}

In flutter we are getting using Dio. I am getting a error using Dio The problem is here that i need to use SSl certificate So i used Dio But now i am getting the below error. Unhandled exception Future is not a subtype of HttpClient.