Fix null errors on conversation holder (#32258) (#32266)

Fix null errors on conversation holder (#32258) (#32266)

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.