Delete Topic destroys the application. HTTP Status 500

1) sign in by any user
2) create topic in any branch
3) click Delete button (to topic) screen 1.png. I see message "delete Post"
4) Confirm delete. i see page of topic with button Answer, without posts. screen 2.png
5) Go to main page. Click to branch (which created the topic). I see error page screen 3.png (and text of trace in file)

expected:
step 3. I should see text "... delete Topic?"
step 4. After removing of topic i should see branch page.