Conflict with Gutenberg

Hi,

It seems that your plugin causes a conflict with Gutenberg. It adds some script before opening <doctype> what leads the browser to go into quirks mode. In Quirks mode the metaboxes are displayed wrongly:

https://github.com/WordPress/gutenberg/pull/12575
https://github.com/WordPress/gutenberg/issues/11378#issuecomment-441840688

I got the following script at the beginning of my HTML under gutenberg editor page in dashboard:

<script>
    if ( !window['tw_gb_wdi'] ) {
      window['tw_gb_wdi'] = {};
    }
    if ( !window['tw_gb_wdi']['tw/wdi'] ) {
      window['tw_gb_wdi']['tw/wdi'] = {
        title: 'Instagram WD',
        iconUrl: 'https://localhost/test/wp-content/plugins/wd-instagram-feed/images/insta_2.svg',
        iconSvg: {
          width: '30',
          height: '30',
          src: 'https://localhost/test/wp-content/plugins/wd-instagram-feed/images/insta.svg'
        },
        data: '{"shortcode_prefix":"wdi_feed","inputs":[{"type":"select","id":"wdi_id","name":"wdi_id","shortcode_attibute_name":"id","options":[{"id":"1","name":"Instagram footer"}]}]}',
      };
    }
  </script>