JsonUnsupportedObjectError (Converting object to an encodable object failed Error in flutter

Question

I am facing problem while trying to encode Json data to a List of String

CartRepo.dart

<pre> <code>

<pre> <code>
import 'dart:convert';
import 'package:ecommerceapp/models/cart_model.dart';
import 'package:ecommerceapp/utils/app_constants.dart';
import 'package:shared_preferences/shared_preferences.dart';

class CartRepo {
  final SharedPreferences sharedPreferences;
  CartRepo({required this.sharedPreferences});

  List<String> cart = [];

  void addToCartList(List<CartModel> cartList) async {
    cart = [];
    cartList.forEach((element) {
      return cart.add(jsonEncode(element.toJson()));
    });
    sharedPreferences.setStringList(AppConstants.CART_LIST, cart);
    getCartList();
  }

  List<CartModel> getCartList() {
    List<String> carts = [];

    if (sharedPreferences.containsKey(AppConstants.CART_LIST)) {
      carts = sharedPreferences.getStringList(AppConstants.CART_LIST)!;
    }

    List<CartModel> cartList = [];

    carts.forEach(
        (element) => cartList.add(CartModel.fromJson(jsonDecode(element))));

    return cartList;
  }
}

CartModel.dart

import 'package:ecommerceapp/models/popular_products.dart';

class CartModel {
  int? id;
  String? name;
  int? price;
  String? img;
  int? quantity;
  bool? isExist;
  String? time;
  ProductModel? product;

  CartModel({
    this.id,
    this.name,
    this.price,
    this.img,
    this.quantity,
    this.isExist,
    this.time,
    this.product,
  });

  CartModel.fromJson(Map<String, dynamic> json) {
    this.id = json['id'];
    this.name = json['name'];
    this.price = json['price'];
    this.img = json['img'];
    this.quantity = json['quantity'];
    this.isExist = json['isExist '];
    this.time = json['time'];
    this.product = ProductModel.fromJson(json['product']);
  }

  Map<String, dynamic> toJson() {
    return {
      "id": this.id,
      "name": this.name,
      "price": this.price,
      "img": this.img,
      "quantity": this.quantity,
      "isExist": this.isExist,
      "time": this.time,
      "product": this.product!.toJson,
    };
  }
}
<pre> <code>

PopularProducts.dart

<pre> <code>
class Product {
  int? _totalSize;
  int? _typeId;
  int? _offset;
  List<ProductModel> _products = [];
  List<ProductModel> get products => _products;

  Product(
      {required totalSize,
      required typeId,
      required offset,
      required products}) {
    this._totalSize = totalSize;
    this._typeId = typeId;
    this._offset = offset;
    this._products = products;
  }

  Product.fromJson(Map<String, dynamic> json) {
    _totalSize = json['total_size'];
    _typeId = json['type_id'];
    _offset = json['offset'];
    if (json['products'] != null) {
      _products = <ProductModel>[];
      json['products'].forEach((v) {
        _products.add(ProductModel.fromJson(v));
      });
    }
  }
}

class ProductModel {
  int? id;
  String? name;
  String? description;
  int? price;
  int? stars;
  String? img;
  String? location;
  String? createdAt;
  String? updatedAt;
  int? typeId;

  ProductModel(
      {this.id,
      this.name,
      this.description,
      this.price,
      this.stars,
      this.img,
      this.location,
      this.createdAt,
      this.updatedAt,
      this.typeId});

  ProductModel.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    name = json['name'];
    description = json['description'];
    price = json['price'];
    stars = json['stars'];
    img = json['img'];
    location = json['location'];
    createdAt = json['created_at'];
    updatedAt = json['updated_at'];
    typeId = json['type_id'];
  }

  Map<String, dynamic> toJson() {
    return {
      "id": id,
      "name": name,
      "price": price,
      "img": img,
      "location": location,
      "createdAt": createdAt,
      "updatedAt": updatedAt,
      "typeId": typeId,
    };
  }
}
<pre> <code>

Error :

Unhandled Exception:Converting object to an encodable object