Describe the bug
As admin, I can report a group but I cannot block it. When executing the action it returns a server error.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'https://try.decidim.org/' and log in as administrator
2. Search for a group (e.g. https://try.decidim.org/profiles/carroll_price/members) and report it
3. Click on block from the admin panel (https://try.decidim.org/admin/moderated_users)
4. See error

Expected behavior
I expect to be able to report and block groups.

Screenshots
https://www.awesomescreenshot.com/video/10918019?key=1436cda730d0579ae8cbb9dab248677d

Extra data (please complete the following information):

• Device: Desktop
• Device OS: Ubuntu
• Browser: Chrome
• Decidim Version: 0.26.1 (also happens in 0.27.0.rc1)
• Decidim installation: MetaDecidim and Try