Describe the bug
When inviting an Existing Participant to a Conference.
The user go into the letter_opener and click Subscribe to Conference link it opens the email in Rich format.
Then there is an error displayed after clicking the Subscribe to Conference link in the invitation mail in rich format in the letter_opener.
But when refreshing the application page with the user who was invited, there is a message saying that the user have succesfully joined the conference.

To Reproduce
Steps to reproduce the behavior:

1. Create a Conference and publish it
2. Create a Registration Type
3. On the Invite page, select Existing Participant and fill the form
4. Check the status it said that the invite is sent.
5. Go into the letter_opener and select the Conference invitation email.
6. Click on the Subscribe to Conference link
7. The mail open in Rich format
8. Click on the Subscribe to Conference link and See the error from the email from the letter_opener
9. Refresh the application home page with the user you just invited to the conference logged in and it says the have succesfully joined the conference

Expected behavior
When clicking the Subscribe to Conference link in the email we should be redirected to the application page with that user and see the message leting him know he has joined the conference. Just like the other invitations works.

Screenshots
When filling the invite form:

To see the status:

The invitation email in the letter_opener:

After clicking the Subscribe to Conference link the 1st time it open the mail in this format:

The error saying : Validation failed: User is not available

The home page of the invited user:

Stacktrace

Extra data (please complete the following information):

• Device OS: Windows 10
• Browser: Chrome
• Decidim Version: 0.25.2
• Decidim installation: Docker