Fix Traceback from Scenario Tree

[soininen]

This PR fixes a Traceback that would appear when selecting the "Type new scenario name here..." item in Scenario tree.

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 70.26%. Comparing base (6409f80) to head (4416ea7).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3247      +/-   ##
==========================================
- Coverage   70.26%   70.26%   -0.01%     
==========================================
  Files         204      204              
  Lines       39127    39129       +2     
  Branches     4722     4723       +1     
==========================================
  Hits        27494    27494              
- Misses      10334    10335       +1     
- Partials     1299     1300       +1     

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