JSON Serialization Issue: The argument type 'Tracks' can't be assigned to the parameter type 'Map<String, dynamic>'

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_gmail_redesign/titles.dart';
//import 'package:flutter_gmail_redesign/widget/header.dart';
//import 'package:flutter_gmail_redesign/widget/mails.dart';


void main() {
  runApp(MaterialApp(
    theme: ThemeData.dark(),
    debugShowCheckedModeBanner: false,
    home: TrackList(),
  ));
}

primary_mail.dart

import 'package:flutter/material.dart';

class PrimaryMail extends StatelessWidget {
  final IconData iconData;
  final String title;
//  final String msg;
  final String count;
  final Color colors;
  PrimaryMail(
      {@required this.iconData,
      @required this.title,
//      @required this.msg,
      @required this.count,
      @required this.colors});

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.fromLTRB(30, 16, 16, 16),
      child: Row(
        mainAxisSize: MainAxisSize.max,
        children: <Widget>[
          Icon(
            this.iconData,
            color: this.colors,
          ),
          Expanded(
            child: Padding(
              padding: const EdgeInsets.symmetric(horizontal: 20),
              child: Column(
                mainAxisSize: MainAxisSize.min,
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  Text(
                    this.title,
                    style: TextStyle(
                        color: Colors.black,
                        fontSize: 17,
                        fontFamily: 'GoogleMedium'),
                  ),
//                  Text(
//                    this.msg,
//                    style: TextStyle(
//                        color: Colors.black87,
//                        fontSize: 15,
//                        fontFamily: 'GoogleRegular'),
//                  ),
                ],
              ),
            ),
          ),
          Container(
            padding: EdgeInsets.symmetric(horizontal: 8, vertical: 5),
            child: Text(
              this.count,
              textAlign: TextAlign.center,
              style: TextStyle(fontSize: 12, fontFamily: 'GoogleMedium'),
            ),
            decoration: BoxDecoration(
                color: this.colors,
                shape: BoxShape.rectangle,
                borderRadius: BorderRadius.circular(20)),
          )
        ],
      ),
    );
  }
}

titles.dart

import 'dart:async';
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:community_material_icon/community_material_icon.dart';
import 'package:flutter/services.dart';
import 'package:flutter_gmail_redesign/Tracks.dart';
//import 'package:flutter_gmail_redesign/widget/header.dart';
//import 'package:flutter_gmail_redesign/widget/mails.dart';
import 'package:flutter_gmail_redesign/widget/primary_mail.dart';
import 'package:outline_material_icons/outline_material_icons.dart';
import 'dart:math';

class TrackList extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _TrackListState();
}

class _TrackListState extends State<TrackList> {
  var tracks = const [];

  Future loadTrackList() async {
    String content = await rootBundle.loadString('data/titles.json');
    List<Tracks> collection = json.decode(content);
    List<Tracks> _tracks = collection.map((json) => Tracks.fromJson(json)).toList();

    setState(() {
      tracks = _tracks;
    });
  }

  void initState() {
    loadTrackList();
    super.initState();
  }

  @override
  Widget build(BuildContext context) => Scaffold(
        backgroundColor: Colors.white,
        floatingActionButton: FloatingActionButton(
          backgroundColor: Colors.white,
          onPressed: () {},
          child: Icon(
            CommunityMaterialIcons.plus,
            color: Colors.black,
          ),
        ),
        body: ListView.builder(
          itemCount: tracks.length,
          itemBuilder: (BuildContext context, int index) {
            var rnd = new Random();
            var totalUsers = rnd.nextInt(600);
            Tracks trackTitles = tracks[index];
            return PrimaryMail(
              iconData: OMIcons.supervisorAccount,
              title: trackTitles.title,
//               msg: 'Subtitle Went Here, originally msg on PrimaryMail.dart',
              count: "$totalUsers active",
              colors: Colors.lightBlueAccent,
            );
          },
        ),
      );
}

titles.json

[{
  "title": "BCS Examination",
  "subtitle": "Something"
},
  {
    "title": "Dhaka University Admission"
  },
  {
    "title": "Khulna University Admission"
  },
  {
    "title": "Chottogram University Admission"
  },
  {
    "title": "Potato Job Exam"
  },
  {
    "title": "Private Job Exam"
  }
]

Tracks.dart

class Tracks{
  final String title;
  final String subtitle;

  Tracks(this.title, this.subtitle);


  Tracks.fromJson(Map<String, dynamic> json) :
      title = json['title'],
      subtitle = json['subtitle'];
}