No feedback when saving sitemenuitem fails

In the CM, when my client tries to save a sitemenuitem that has a 'sitemap-verwijzing' that starts with a slash, a node name can't be encoded and an exception is thrown. However, there is no feedback at all. Their input has vanished for no visible reason.

Using the console, it is allowed to use a reference starting with a slash and everything works fine. So now in the console I created working configuration that my client can't save via the CMS anymore.

Please make the CM accept references starting with a slash and not swallow exceptions.

See screenshots.