Return the thumbnail meta data for getter and setter

Change you constructor to this

public function __construct($post)
{
    $this->set_bio($post->post_content);
    $thumbnail = get_the_post_thumbnail_url($post->ID);
    $this->set_thumbnail($thumbnail);
}

and remove thumbnail code from init method.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)