Error 500 when manipulating with moderating groups

1. Create a new user (f.ex. 1234)
2. Go to Admin Panel and create a new group, having the same name as a new user from Step 1 (f.ex.1234).
3. Add user 1234 to the new group named 1234.
4. Choose any section, create a new branch, give it the same name(1234).
5. Assign the group 1234 as a moderating group of the branch 1234.
6. Grant the group 1234 the following permissions for this branch: VIEW_TOPIC, CLOSE_TOPIC, CREATE_POSTS,DELETE_OWN_POSTS, DELETE_OTHERS_POSTS.
7. Go to Forum, create a new topic in the new branch, the name of the topic should be the same (1234)
8. Create one more new group (having any name), add a User 1234 to the new group
9. Assign this group as a moderating to the branch 1234
10. Delete the first goup (1234)
11. Go to the main page of the Forum (you should be logged in)

As a result
error 500 is shown
Expected result
the main page should be shown