Error 500 in Recent Activity when delete group that has View_Topic permission

Preconditions:
1.Branch "Test" is created;
2.Group1 granted permissions View_Topic for branch "Test"
3. User1 is a member of Group1
4. Group "Registered Users" doesn`t have any permissions to "Test" branch

How to reproduce:
1. Login to Forum as User1
2. Create topic "Test" in Branch "Test"
3. Click on Recenta activity link in forum main page
4. Verify displaying topic Test in a list of recent activities
5. Go to Admin panel
6. Delete Group1, in warning dialog give permissions to group "Registered Users"
7. Go to Forum
8. Refresh "Recent Activity" page

AR: Error 500
ER: No errors

NOTE: reproduced when deleted group is the only group that has View_Topics permissions. Not reproduced when two or more groups have View_Topic permission and no problem when one group is deleted