It is possible to create a user without a user group or you can delete a user group and save the user.
But an error appears when trying to remove a user from a user group. Though it is possible to create a user group without a user.

Steps to reproduce:
1. Create a new user with a user group;
2. Open user groups and try to remove the user from this group -> an error appears;
3. Open the user and remove the user group -> user group removed.

Result:
The error appears when trying to remove a user from a user group.

Expected:
Should be the same behavior in both cases.