Fix opening URL selector in importer causes Traceback

[PiispaH]

Fixed typo

Fixes spine-tools/Spine-Toolbox#2372

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: 1 lines in your changes are missing coverage. Please review.

Comparison is base (3f20757) 59.73% compared to head (1964b7f) 59.72%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #165 +/- ## =========================================== - Coverage 59.73% 59.72% -0.01% =========================================== Files 193 193 Lines 17546 17546 Branches 2847 2847 =========================================== - Hits 10481 10480 -1 Misses 6502 6502 - Partials 563 564 +1 ``` | [Files](https://app.codecov.io/gh/spine-tools/spine-items/pull/165?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools) | Coverage Δ | | |---|---|---| | [...ine\_items/importer/widgets/import\_editor\_window.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/165?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvaW1wb3J0ZXIvd2lkZ2V0cy9pbXBvcnRfZWRpdG9yX3dpbmRvdy5weQ==) | `42.16% <0.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/spine-tools/spine-items/pull/165/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools)

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