I was able to reproduce this on two different installations running 2.5.0.beta4 e81a4048e3. You can even reproduce it here on meta.discourse.org!
- Go to a group’s page like
/g/group_name
- Click on
Activity
(or just go to https://meta.discourse.org/g/theme_creator/activity/posts) - Click on a topic name
- See the error
It generates an url like this one:
https://meta.discourse.org/g/theme_creator/activity/undefined/t/self-hosting-for-free-with-oracle-cloud/139184/13
Same thing happens under “Mentions”
Searched and found this previous bug, which isn’t the same but maybe helpful? Following a direct link to group activity results in 400 error