JavaScript errors while pasting widgets on a dashboard

This can be reproduced if window gets resized immediately after hitting Paste button on an editable dashboard (obviously, some widget must be copied first). After hitting Paste button, dashboard.widget.sanitize action gets requested through the AJAX, and while the request is in progress, a widget imitation (a placeholder) is displayed on dashboard. This placeholder is stored like real widget inside the JavaScript, thus causing errors whether the code expects real widgets.