Display not found page after getting null from API

[dominik-zeglen]

Resolves #3105

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[dominik-zeglen]

@pwgryglak we need some design for this

[codecov]

Codecov Report

Merging #3463 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3463   +/-   ##
=======================================
  Coverage   89.87%   89.87%           
=======================================
  Files         241      241           
  Lines       13103    13103           
  Branches     1322     1322           
=======================================
  Hits        11776    11776           
  Misses        922      922           
  Partials      405      405

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