flutter building listview with response from restapi

You can copy paste run full code below
Your json string is PartnerList not List<PartnerList>
You can use Future<PartnerList> and use return Future.value
code snippet

Future<PartnerList> fetchPartner() async {
    ...

    return Future.value(partnerListFromMap(response.body));

working demo

full code

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

import 'dart:convert';

PartnerList partnerListFromMap(String str) =>
    PartnerList.fromMap(json.decode(str));

String partnerListToMap(PartnerList data) => json.encode(data.toMap());

class PartnerList {
  PartnerList({
    this.data,
  });

  List<Datum> data;

  factory PartnerList.fromMap(Map<String, dynamic> json) => PartnerList(
        data: List<Datum>.from(json["data"].map((x) => Datum.fromMap(x))),
      );

  Map<String, dynamic> toMap() => {
        "data": List<dynamic>.from(data.map((x) => x.toMap())),
      };
}

class Datum {
  Datum({
    this.id,
    this.partnerTypeId,
    this.active,
    this.name,
    this.shortname,
    this.firstname,
    this.lastname,
    this.addressId,
    this.phoneId,
    this.countryId,
    this.email,
    this.web,
    this.infos,
    this.note,
    this.createdAt,
    this.updatedAt,
    this.updatedBy,
  });

  int id;
  int partnerTypeId;
  bool active;
  String name;
  String shortname;
  dynamic firstname;
  dynamic lastname;
  int addressId;
  dynamic phoneId;
  String countryId;
  String email;
  String web;
  Infos infos;
  dynamic note;
  DateTime createdAt;
  DateTime updatedAt;
  dynamic updatedBy;

  factory Datum.fromMap(Map<String, dynamic> json) => Datum(
        id: json["id"],
        partnerTypeId: json["partnerTypeId"],
        active: json["active"],
        name: json["name"],
        shortname: json["shortname"],
        firstname: json["firstname"],
        lastname: json["lastname"],
        addressId: json["addressId"] == null ? null : json["addressId"],
        phoneId: json["phoneId"],
        countryId: json["countryId"],
        email: json["email"],
        web: json["web"] == null ? null : json["web"],
        infos: Infos.fromMap(json["infos"]),
        note: json["note"],
        createdAt: DateTime.parse(json["createdAt"]),
        updatedAt: DateTime.parse(json["updatedAt"]),
        updatedBy: json["updatedBy"],
      );

  Map<String, dynamic> toMap() => {
        "id": id,
        "partnerTypeId": partnerTypeId,
        "active": active,
        "name": name,
        "shortname": shortname,
        "firstname": firstname,
        "lastname": lastname,
        "addressId": addressId == null ? null : addressId,
        "phoneId": phoneId,
        "countryId": countryId,
        "email": email,
        "web": web == null ? null : web,
        "infos": infos.toMap(),
        "note": note,
        "createdAt": createdAt.toIso8601String(),
        "updatedAt": updatedAt.toIso8601String(),
        "updatedBy": updatedBy,
      };
}

class Infos {
  Infos();

  factory Infos.fromMap(Map<String, dynamic> json) => Infos();

  Map<String, dynamic> toMap() => {};
}

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

class MyApp extends StatelessWidget {
  @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> {
  Future<PartnerList> _future;

  Future<PartnerList> fetchPartner() async {
    String jsonString = '''
    {"data":[{"id":1,"partnerTypeId":1,"active":true,"name":"Covin - AINO UG","shortname":"Covin","firstname":null,"lastname":null,"addressId":1,"phoneId":null,"countryId":"in","email":"[email protected]","web":"www.covin.in","infos":{},"note":null,"createdAt":"2021-03-06 23:30:30","updatedAt":"2021-03-06 23:30:30","updatedBy":null},{"id":41,"partnerTypeId":100,"active":true,"name":"Tester IND","shortname":"T.","firstname":null,"lastname":null,"addressId":null,"phoneId":null,"countryId":"IN","email":"email@tester","web":null,"infos":{},"note":null,"createdAt":"2021-03-06 23:32:02","updatedAt":"2021-03-07 01:25:06","updatedBy":null},{"id":42,"partnerTypeId":100,"active":true,"name":"MIT Charl","shortname":"KITA Ch.","firstname":null,"lastname":null,"addressId":null,"phoneId":null,"countryId":"IN","email":"kisa1@kol","web":null,"infos":{},"note":null,"createdAt":"2021-03-06 23:32:42","updatedAt":"2021-03-07 01:25:08","updatedBy":null}]}
    ''';
    final response = http.Response(jsonString, 200);

    if (response.statusCode == 200) {
      // If the server did return a 200 OK response,
      // then parse the JSON.
      print(response.body);

      return Future.value(partnerListFromMap(response.body));
    } else {
      // If the server did not return a 200 OK response,
      // then throw an exception.
      throw Exception('Failed to load PartnerList');
    }
  }

  @override
  void initState() {
    _future = fetchPartner();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: FutureBuilder(
            future: _future,
            builder: (context, AsyncSnapshot<PartnerList> snapshot) {
              switch (snapshot.connectionState) {
                case ConnectionState.none:
                  return Text('none');
                case ConnectionState.waiting:
                  return Center(child: CircularProgressIndicator());
                case ConnectionState.active:
                  return Text('');
                case ConnectionState.done:
                  if (snapshot.hasError) {
                    return Text(
                      '${snapshot.error}',
                      style: TextStyle(color: Colors.red),
                    );
                  } else {
                    return ListView.builder(
                        itemCount: snapshot.data.data.length,
                        itemBuilder: (context, index) {
                          return Card(
                              elevation: 6.0,
                              child: Padding(
                                padding: const EdgeInsets.only(
                                    top: 6.0,
                                    bottom: 6.0,
                                    left: 8.0,
                                    right: 8.0),
                                child: Row(
                                  crossAxisAlignment: CrossAxisAlignment.start,
                                  children: <Widget>[
                                    Text(snapshot.data.data[index].id
                                        .toString()),
                                    Spacer(),
                                    Text(
                                      snapshot.data.data[index].name,
                                    ),
                                  ],
                                ),
                              ));
                        });
                  }
              }
            }));
  }
}