Mostrar índice indefinido: carrinho quando meu carrinho de compras está vazio

Aqui está todo o meu código.
checkout.php
<? php session start ();
requer uma vez (“../ dataAccessLayer / dalSession.php”);
requer uma vez (“../ dataAccessLayer / dalProduct.php”);
requer uma vez (“../ dataAccessLayer / dalService.php”);
?>
<? php include (“../ header2.php”);?>
<br>
<div class = “w3-content” style = “max-width: 1400px; margin-top: 40px;”>

<! – The Grid ->
<div class = “w3-row-padding”>

<div class = “w3-container w3-card-2 w3-white w3-margin-bottom”>

<br>
<div class = “container”>

<h1>Shopping Cart</h1><hr>
<table class="table table-striped table-hover table-bordered">
    <tbody>
    <tr>
        <th>Product</th>
        <th>Price</th>
        <th>Vat</th>
        <th>Discount</th>
        <th>Quantity</th>
        <th>Total Price</th>
        <th>Delete</th>
    </tr> 

    <?php

    if(isset($_POST['sub']))
    {

        for($i=0; $i<count($_SESSION['cart']); $i++)
        {
            //print "for loop in";
            if(isset($_POST['qty_' . $_SESSION['cart'][$i]]))
            {

                //print $_POST['qty_' . $_SESSION['cart'][$i]];
                $_SESSION['qty'][$i] = $_POST['qty_'.$_SESSION['cart'][$i]]; 
            }
        }

    }



    $total = 0;
    //$_SESSION['cart']=[];
   //if(isset($_POST['sub'])){      
        for($i=0; $i<count($_SESSION['cart']); $i++) //line 57
        {
            $p = new Product();
            $p->product_id = $_SESSION['cart'][$i];
            $r = $p->SelectById();
            print "<tr>";

imprimir “<td align = ” center “”> </td>””;
echo “”<td align = “” center “”> $ r [2]””