Fix exception in 0002 migration

[Mogost]

@sergei-iurchenko could you please check this improvement?

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.39%. Comparing base (ef24fcb) to head (c72c72b).
Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
...onstance/migrations/0002_migrate_from_old_table.py	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #579      +/-   ##
==========================================
- Coverage   89.92%   85.39%   -4.53%     
==========================================
  Files          31       21      -10     
  Lines        1032      719     -313     
  Branches      104      125      +21     
==========================================
- Hits          928      614     -314     
- Misses         69       70       +1     
  Partials       35       35              

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