Fix Traceback when disconnecting from an SQLite source in Importer specification editor

[soininen]

Fixes spine-tools/Spine-Toolbox#2440

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0a48db6) 60.23% compared to head (8de668e) 60.16%.

Files	Patch %	Lines
spine_items/importer/connection_manager.py	77.77%	2 Missing :warning:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #173 +/- ## =========================================== - Coverage 60.23% 60.16% -0.08% =========================================== Files 194 194 Lines 17680 17678 -2 Branches 2887 2886 -1 =========================================== - Hits 10650 10636 -14 - Misses 6460 6470 +10 - Partials 570 572 +2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.