fix(editor): add index checking to selected elements array

[L-Sun]

Close https://toeverything.sentry.io/issues/6187666983/?alert_rule_id=15031714&alert_type=issue&notification_uuid=fe3f4b9d-1e86-420e-a715-c3ca0fa932e4&project=4506307500179456&referrer=slack

[graphite-app]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 51.64%. Comparing base (597b631) to head (b51de2a).
Report is 1 commits behind head on canary.

Files with missing lines	Patch %	Lines
.../root-block/widgets/element-toolbar/lock-button.ts	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           canary    #9453      +/-   ##
==========================================
- Coverage   51.83%   51.64%   -0.19%     
==========================================
  Files        2154     2154              
  Lines       97036    97039       +3     
  Branches    16208    16173      -35     
==========================================
- Hits        50297    50120     -177     
- Misses      45410    45588     +178     
- Partials     1329     1331       +2     

Flag	Coverage Δ
server-test	78.07% <ø> (-0.65%)	⬇️
unittest	31.10% <0.00%> (+0.05%)	⬆️

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[Saul-Mirone]

Merge activity

• Dec 31, 3:04 AM EST: The merge label 'merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Dec 31, 3:04 AM EST: A user added this pull request to the Graphite merge queue.
• Dec 31, 3:18 AM EST: A user merged this pull request with the Graphite merge queue.