[BUG] Error: Add member to chat

[FelberMartin]

Describe the bug

When trying to add a new member to a chat, an error occurs.

To Reproduce
Steps to reproduce the behavior:

1. Login into TS0 with test_user_1
2. Go to General Testing Course > Communication > Group Chats > #error-add-member
3. Click settings icon
4. Click "Add members"
5. Search for a user that is not yet in the group, eg "test user 4"
6. Click "Add"
7. Error dialog appears

Expected behavior
No error dialog should be shown and the user should be added to the chat.

Screenshots

Next steps

• Investigate where error root cause is
• Fix error
• Ensure proper behaviour with tests