Trying to echo a button when user_id == user_id , PHP CodeIgniter

I'm trying to make a button so when the users -> user_id == the same as the products-> user_id echo a button but its not working. So only when the product has the same user_id as the session user_id , I want to echo a button with a link to another page but its not working.

This is what I tried to do:

<div class="cadeau_bewerken">
        <?php if ( $_SESSION['user_id'] ==  $product["user_id"]): ?>
    <a class="btn btn-primary" href="<?php echo base_url() ?>/KdGwController/details_bewerken/<?php echo $product->product_id; ?>"> Cadeau bewerken </a>
<?php endif; ?>
    </div>

I do see a button but its not a link and not working and I'm getting this error:

A PHP ERROR WAS ENCOUNTERED
Severity: Notice
Message: Trying to get property of non-object
Filename: views/details.php
Line number: 54

This is line number 54 in details.php:

<a class="btn btn-primary" href="<?php echo base_url() ?>/KdGwController/details_bewerken/<?php echo $product->product_id; ?>"> Cadeau bewerken </a>

And yes I defined $product because on the same page I'm able to echo the name for example like this:

<?php echo $product['product_name']; ?>