Fix restoring UI in Database editor

[soininen]

This PR fixes a bug where DB editor could not be opened.

Fixes #3226

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.05%. Comparing base (6016b42) to head (6a2ccf5).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...toolbox/spine_db_editor/widgets/spine_db_editor.py	54.54%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3227      +/-   ##
==========================================
- Coverage   69.05%   69.05%   -0.01%     
==========================================
  Files         199      199              
  Lines       38653    38656       +3     
  Branches     4632     4634       +2     
==========================================
+ Hits        26692    26693       +1     
- Misses      10652    10654       +2     
  Partials     1309     1309              

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