Erro <!DOCTYPE html> em flutter | Mobile

Estava tentando fazer uma ligação flutter com firebase realtime e agora esta aparecendo isso

I/flutter (18636): FormatException: Unexpected character (at line 2, character 1)
I/flutter (18636): <!DOCTYPE html>
I/flutter (18636): ^

Meus arquivos principais: UserApi

import 'dart:convert' as convert;
import 'package:fluter_crud/models/user.dart';

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

class UserApi {
  static Future<List<User>> getUser() async {
    final String url = 'https://app-armario.firebaseio.com/';
    var response = await http.get(url);
    String json = response.body;
    List list = convert.json.decode(json);
    final user = List<User>();

    for (Map map in list) {
      User c = User.fromJson(map);
      user.add(c);
    }

    print(user);
    return user;
  }
}

Classe do User

import 'package:flutter/material.dart';

class User {
  String id;
  String name;
  String email;
  String avatarUrl;

  User({
    this.id,
    @required this.name,
    @required this.email,
    @required this.avatarUrl,
  });

  User.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    name = json['name'];
    avatarUrl = json['avatarUrl'];
    email = json['email'];
  }

  Map<String, User> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['id'] = this.id;
    data['name'] = this.name;
    data['email'] = this.email;
    data['avatarUrl'] = this.avatarUrl;
    return data;
  }
}

Parte de criar a lista:

import 'package:fluter_crud/components/user_tile.dart';
import 'package:fluter_crud/models/user.dart';
import 'package:fluter_crud/provider/users.dart';
import 'package:fluter_crud/routes/app_routes.dart';
import 'package:fluter_crud/utils/User_api.dart';

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

class UserList extends StatelessWidget {
  //Future<void> _refreshUsers(BuildContext context) {}
  @override
  Widget build(BuildContext context) {
    //final Users usersData = Provider.of(context);
    return Scaffold(
      appBar: AppBar(
        title: Text('Lista de Usuários'),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.add),
            onPressed: () {
              Navigator.of(context).pushNamed(
                AppRoutes.USER_FORM,
              );
            },
          )
        ],
      ),
      body: teste(),
    );
  }

  teste() {
    Future<List<User>> future = UserApi.getUser();
    return FutureBuilder(
      future: future,
      builder: (context, snapshot) {
        if (!snapshot.hasError) {
          print(snapshot.error);
          return Center(child: Text("Erro ao carregar banco de dados"));
        }
        if (!snapshot.hasData) {
          print(snapshot.error);
          print(snapshot.data);
          return Center(
            child: CircularProgressIndicator(),
          );
        }
        List<User> users = snapshot.data;
        return _listView(users);
      },
    );
  }

  Container _listView(List<User> users) {
    return Container(
      child: ListView.builder(
        itemCount: users != null ? users.length : 0,
        itemBuilder: (ctx, i) {
          User u = users[i];
          return UserTile(u);
        },
      ),
    );
  }
}
//RefreshIndicator( onRefresh: () => _refreshUsers(context),child:
//itemBuilder: (ctx, i) => UserTile(usersData.byIndex(i),