ReferenceError: $ is not defined

Your widget has Underscore.js/LoDash.js as dependency.

You can get them here: underscore, lodash

Try prepending this to your code, so you can see if it works:

<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/lodash.js/0.10.0/lodash.min.js"></script>

i was facing the same problem in the wp-admin section of the site. I enqueued the underscore script cdn and it fixed the problem.

function kk_admin_scripts() {
    wp_enqueue_script('underscore', '//cdnjs.cloudflare.com/ajax/libs/lodash.js/0.10.0/lodash.min.js' );
}
add_action( 'admin_enqueue_scripts', 'kk_admin_scripts' );