Add optional chaining and explicitely return "Not found" page

[dominik-zeglen]

I want to merge this change because it adds optional chaining and fixes not found error handling.

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. Translated strings are extracted to .pot file.
4. Number of API calls is optimized.
5. The changes are tested.
6. Type definitions are up to date.
7. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #408 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #408   +/-   ##
=======================================
  Coverage   72.48%   72.48%           
=======================================
  Files         639      639           
  Lines        7120     7120           
  Branches     1180     1180           
=======================================
  Hits         5161     5161           
  Misses       1718     1718           
  Partials      241      241

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