Avoid possible UnboundLocalError for end_event

[berland]

Issue
Resolves end_event possibly being unbound

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• 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 91.50%. Comparing base (5717e4d) to head (b04bd19).
Report is 1 commits behind head on version-11.0.

Additional details and impacted files

@@              Coverage Diff              @@
##           version-11.0    #9028   +/-   ##
=============================================
  Coverage         91.50%   91.50%           
=============================================
  Files               343      343           
  Lines             21276    21277    +1     
=============================================
+ Hits              19468    19470    +2     
+ Misses             1808     1807    -1     

Flag	Coverage Δ
cli-tests	38.99% <100.00%> (+0.01%)	⬆️
gui-tests	72.42% <100.00%> (+<0.01%)	⬆️
performance-tests	49.30% <100.00%> (-0.02%)	⬇️
unit-tests	80.11% <100.00%> (-0.01%)	⬇️

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