fix#32258
Errors in the issue was due to unhandled null check. so i fixed it.
### Detailed description for Issue & Fix
To reproduce that issue, the comment must be deleted on Conversation
tab.
#### Before Delete
<img width="1032" alt="image"
src="https://github.com/user-attachments/assets/72df61ba-7db6-44c9-bebc-ca1178dd27f1">
#### After Delete (AS-IS)
<img width="1010" alt="image"
src="https://github.com/user-attachments/assets/36fa537e-4f8e-4535-8d02-e538c50f0dd8">
gitea already have remove logic for `timeline-item-group`, but because
of null ref exception the later logic that removes `timeline-item-group`
could be not be called correctly.
(cherry picked from commit 603fca1e27bc29c1e700cc1bd284eb619d2436c8)
c163bf6fb5