Wiki sidebar cannot be reopened on non-existent wiki pages in the New UI

Summary

When navigating to a wiki page that does not exist, the sidebar reopen control is missing. Users can collapse the sidebar but cannot reopen it.

Steps to Reproduce

1. Navigate to a wiki page URL that does not exist (e.g., -/wikis/non-existent-page
2. The page displays "This page does not exist"
3. Collapse the wiki sidebar
4. Attempt to reopen the sidebar
5. Expected: Sidebar reopen control should appear
6. Actual: No reopen control appears; sidebar remains closed

Current Behavior

• Sidebar collapse works on non-existent wiki pages
• Reopen control does not appear after collapsing
• Users cannot navigate without manually typing a new URL

Expected Behavior

• Sidebar reopen control should be visible and functional on all wiki pages, including error pages for non-existent pages
• Users should be able to collapse and reopen the sidebar regardless of page state

Workaround

Navigate manually to -/wikis/pages to restore sidebar access.

Environment

• GitLab.com