Почему выдает ошибку TypeError: null is not an object?

Почему при добавлении товара в корзину выдает ошибку [Vue warn]: Error in v-on handler: "TypeError: null is not an object (evaluating 'state.cart[item.id]')"?

import Vue from "vue";
import Vuex from "vuex";

let cart = window.localStorage.getItem('cart');
let cartCount = window.localStorage.getItem('cartCount');

Vue.use(Vuex);

export const store = new Vuex.Store({
    state: {
        cart: cart ? JSON.parse(cart) : {},
        cartCount: cartCount ? parseInt(cartCount): 0,
    },
    mutations: {
        addToCart(state, item) {
            let cartItem = state.cart[item.id];
            if(state.cart[item.id]){
                cartItem.quantity++
                cartItem.totalPrice = cartItem.quantity * cartItem.price;
            }
            else{
                state.cart[item.id] = item;
                Vue.set(item, 'quantity', 1);
                Vue.set(item, 'totalPrice', item.price);
            }
            state.cartCount++;
            this.commit('saveCart');
        },

        saveCart(state){
            window.localStorage.setItem('cart', JSON.stringify(state.cart));
            window.localStorage.setItem('cartCount', state.cartCount);
        
        },
    },
})

<template>
<div class="product">
    <div class="product-content">
        <span class="product-title">
            <router-link :to="/product/ + id">{{
                    title
                }}</router-link>
        </span>
        <div class="product-bottom">
            <div class="product-price">
                {{ price }}
            </div>
            <div class="product-button">
                    <button v-on:click="addToCart()">
                        добавить в корзину
                    </button>
            </div>
        </div>
    </div>
</div>
</template>

<script>
export default {
    props: {
        id: {
            type: Number,
            default: "NULL",
        },
        title: {
            type: String,
            default: "NULL",
        },
        price: {
            type: String,
            default: "NULL",
        },
        object: {
            type: Object,
            default: 'NULL'
        }
    },
    methods: {
        addToCart(){

            this.$store.commit('addToCart', this.object);
        },
        
    }
}
</script>