Describe the bug
Creating an event and trying to "Import" parts of an unlisted events by pasting the URL of the unlisted event does not show any UI response (i.e. event is not "shown" / no error is shown, can not proceed).

To Reproduce
Steps to reproduce the behavior:

1. Create an unlisted event with some content, e.g. registration forms, which you want to import into another event.
2. Enter the event you want to import data into, click on the cog-wheel, then choose "Import".
3. Paste the URL from the unlisted event.
4. No event shown / can not proceed (also, no error shown). If there already was an error message present e.g. from a previously entered URL, it remains there.

Expected behavior
Either an error message should be shown (i.e. "can not import from unlisted events"), or the unlisted event should be found correctly and it should be possible to import from it.

Screenshots

Additional context
Listing the unlisted event, without any further changes, the import functionality starts to work.