Unable to add recipients to distribution list

With the attached webapp setting you are unable to add any contact to a distribution list

Steps to reproduce:

• restore setting with Kopano-webapp-admin.py --restore --file broken-dist.json -u user1
• go to contacts
• create new distribution list
• Add new...
• Press Ok or cancel
• nothing happens

Removing the line "distlistexternalmembercontentpanel": [], in de json fixes this.