Fix fetch_more() Traceback after calling refresh_session()

[soininen]

Fixes #287

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

All modified lines are covered by tests :white_check_mark:

Comparison is base (adf1ea5) 76.61% compared to head (cba5a2d) 76.67%. Report is 1 commits behind head on 0.8-dev.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #288 +/- ## =========================================== + Coverage 76.61% 76.67% +0.06% =========================================== Files 73 73 Lines 8936 8936 Branches 1888 1888 =========================================== + Hits 6846 6852 +6 + Misses 1761 1756 -5 + Partials 329 328 -1 ``` | [Files](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/288?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools) | Coverage Δ | | |---|---|---| | [spinedb\_api/db\_mapping\_base.py](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/288?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVkYl9hcGkvZGJfbWFwcGluZ19iYXNlLnB5) | `88.04% <100.00%> (+0.69%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/288/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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