Adding a gadget to an empty dashboard fails with a javascript error

To reproduce:

• create a page, edit in wysiwyg mode and add a dashboard macro to it, save the page
• edit the page in inline mode, to get the dashboard editor
• click on the add gadget button, follow the steps of the wizard
• at the end, click "Insert gadget"
• the gadget is not added to the dashboard, nothing happens
• in the javascript console of the browser an error will be displayed