Unhandled Error!

Trying to get property of non-object

Document	shop:add_review_form
Document type	CMS partial
Line	3
Exception class	Cms_ExecutionException

Code Highlight

<!--
<?= open_form(array('class'=>'custom')) ?>
<input type="hidden" name="product_id" value="<?= $product->id ?>" />
<div class="content twelve columns">
    <? if (isset($review_posted)): ?>
        <p class='flash success'>Your review has been successfully posted.</p>
    <? endif; ?>
 
    <h3>Write a review</h3>
 
    <div class="row">
        <div class="one columns">
            <label>Rating</label>

#	Document	Type
2	shop:add_review_form	CMS partial
1	Product details page.	CMS page