Session superglobal variable [Laravel 5.4]

I have a little problem with the Session variable in Laravel. You see,I fill the Session variable cart, in the last method, but I cannot access to the data from another method, only from the addProduct() method, I do not know what more to do, I have already read the documentation of my version and posts of third persons and nothing works for me.

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Productos;
use Session;

class ShoppingCart extends Controller
{
    public function getAmount(){
        $total = 0;
        $cart = Session::get('cart');
        if($cart == null){
            foreach($cart as $product) {
                $total += Productos::find($product)->precio;
            }
        }
        return $total;
    }
    public function getProducts()
    {
        return Session::get('cart');
    }

    public function addProduct($id)
    {
        if(Session::get('cart') == null){
            $cart = [$id];
            Session::put('cart', $cart);

        }else{
            $cart = array_merge(Session::get('cart'), [$id]);
            Session::put('cart', $cart);
        }
        var_dump(Session::get('cart'));
    }
}

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