bt-source/panel/BTPanel/static/ckeditor/plugins/preview/preview.html

<script>

// Prevent from DOM clobbering.
if ( typeof window.opener._cke_htmlToLoad == 'string' ) {
	var doc = document;
	doc.open();
	doc.write( window.opener._cke_htmlToLoad );
	doc.close();

	delete window.opener._cke_htmlToLoad;
}

</script>