Describe the bug
As a participant when creating a meeting, I see an exception on web development console.

To Reproduce
Steps to reproduce the behavior:

1. Sign in as admin
2. Enable "Participants can create meetings" in the configuration of meetings in a participatory process
3. Open the JavaScript console of your browsers web development tool
4. Go to the meeting creation form of participants
5. See error

Expected behavior
I shouldn't see any exception

Screenshots

Firefox

Chromium

Extra data (please complete the following information):

• Decidim Version: 0.26.0.dev
• Decidim installation: decidim.populate.tools and localhost

Additional context
Detected in #8602 (comment)