====Exeption caught by widgets library ==== Class 'String' has no instance method 'toStringAsFixed'

Question

I am very new to flutter and following a tutorial video on how to create a cart for a mobile application, this has a cart Screen with a widget inside it for calculating a cart total, this mentioned cart total widget relies on another file i named cart controller to calculate a subtotal for each item added to cart and a grand total for all cart items within the cart.

however when i build the app i keep getting an error stating Class 'String' has no instance method 'toStringAsFixed'

I need help on how i can be able to show the total of all items available in the cart

heres my code for the cart controller:

import 'package:get/get.dart';
import 'package:shop_app/models/things.dart';

class CartController extends GetxController{

//Add dictionary to store the products in the cart
  var _things = {}.obs;

  //void AddProduct
  void addProduct(Things things) {
    if (_things.containsKey(things)){
      _things[things] += 1;
    } else {_things[things] = 1;}

    Get.snackbar("Product added!","You have added ${things.title} to the cart",
    snackPosition: SnackPosition.TOP,
    duration: Duration(seconds: 2),
    );
  }
  //void RemoveProduct
  void removeProduct(Things things){
    if (_things.containsKey(things) && _things[things] == 1) {
      _things.removeWhere((key, value) => key == things);
    }
    else {
      _things[things] -= 1;
    }
    Get.snackbar("Product removed!","You have removed ${things.title} from the cart",
      snackPosition: SnackPosition.BOTTOM,
      duration: Duration(seconds: 2),
    );
  }

  //get Products;

  get products => _things;


  //get ProductsSubtotal
  get productsSubtotal => _things.entries
      .map((things) => things.key.price * things.value)
      .toList();

  //get Total
  get total => _things.entries
      .map((things) => things.key.price * things.value)
      .toList()
      .reduce((value, element) => value + element)
      .toStringAsFixed(2);

}

heres my code for the cart total widget that's supposed to display this grand total:

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

import 'package:get/get.dart';
import 'package:shop_app/controllers/cart_controller.dart';

class CartTotal extends StatelessWidget {
  final CartController controller = Get.find();
   CartTotal({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
            padding: const EdgeInsets.symmetric(horizontal: 50),
            child: Row(

                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: [
                  Text(
                    'Total',
                    style: TextStyle(
                      fontSize: 24,
                      fontWeight: FontWeight.bold,
                    ),
                  ),
                   Obx( () =>
                     Text(
                       //
                        '${controller.total}',
                        style: TextStyle(
                          fontSize: 24,
                          fontWeight: FontWeight.bold,
                        ),
                      ),
                   ),

                ],
              ),


      );
  }
}

this is what i'm going for