I'm facing the "TypeError: Cannot read properties of undefined (reading 'find')" issue. I attached the code. Thank you in Advance

       switch (action.type) {
case "CART_ADD_ITEM":
  const newItem = action.payload;
  const existItem = state.cartItems.find(
    (item) => item._id === newItem._id
  );
  const cartItems = existItem
    ? state.cart.cartItems.map((item) =>
        item._id === existItem._id ? newItem : item
      )
    : [...state.cart.cartItems, newItem];
  return {
    ...state,
    cart: {
      ...state.cart,
      cartItems,
    },
  };