Steps to reproduce:
After customer request I reproduced it in studio demo site.
I only had an older (1.7.2) studio demo, so I updated it to the current version.
steps to reproduce are based on studio demo, however they should apply to platform demo in an similar way.

1. Extend Contenttype "Blog post" with a single relation and a multi relation field.
2. Create or edit an existing Blog post
3. Make an embed into the richtext field, go to the create tab, select "Blog Post"
4. Try to use one of the relation fields

Nothing happens, no JS Error no UDW :/