Avoid possible UnboundLocalError for end_event

[berland]

The error has been observed to occur in the log. Not trying to display further error messages to why it is None.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.81%. Comparing base (776174b) to head (f8a162f).
Report is 2 commits behind head on version-10.3.

Additional details and impacted files

@@              Coverage Diff              @@
##           version-10.3    #8945   +/-   ##
=============================================
  Coverage         90.81%   90.81%           
=============================================
  Files               342      342           
  Lines             20965    20966    +1     
=============================================
+ Hits              19039    19041    +2     
+ Misses             1926     1925    -1     

Flag	Coverage Δ
gui-tests	75.63% <100.00%> (+<0.01%)	⬆️
integration-tests	53.77% <100.00%> (+<0.01%)	⬆️
unit-tests	68.77% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.