Bugfix: error message text comments thread open with no reviews

Description of change

Added a sentinel in the text comments thread panel to avoid the error and try to display comments only if a review is associated to a media.

Fix #186 (closed)

How to test it

• Load timeline without any reviews
• Open the text comments panel

Checklist

Please check if your MR fulfills the following requirements:

• Unit tests have been added, if needed
• Documentation have been updated, if needed
• Build was run locally and any changes were pushed
• Lint has passed locally without error
• Package.py have been updated, if needed
• CHANGELOG.md have been updated, if needed
• Version numer has been tagged, if needed

FYI: @seblrd