cannot access property of non object

I recently started receiving this error message at the top of my website:

Cannot access property of non object in wp-content/plugins/smart-syntax/includes/functions.php at line 17

I modified the functions.php file like this to avoid getting the error:

    $content = "";
    if(is_object($post) && is_object($comment) && property_exists($post,"post_content") && property_exists($comment,"comment_content") ){
        $content      = $post->post_content . $comment->comment_content;
    }