Fix database locking issues in DB editor

[soininen]

This PR fixes two cases of SQLite objects created in a thread can only be used in that same thread.

No associated issue

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

All modified and coverable lines are covered by tests ✅

Project coverage is 67.20%. Comparing base (f129273) to head (954cf69).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3040      +/-   ##
==========================================
- Coverage   67.21%   67.20%   -0.02%     
==========================================
  Files         193      193              
  Lines       37252    37254       +2     
  Branches     4472     4472              
==========================================
- Hits        25038    25035       -3     
- Misses      10942    10949       +7     
+ Partials     1272     1270       -2     

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