Fix null handling in updateUndoRedoState method

[TheCiosiek]

The updateUndoRedoState method now properly handles cases where undoChange is null, preventing null pointer exceptions.

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See the Progress checklist in the description for automated requirements.

[openjdk]

⚠️ @TheCiosiek a branch with the same name as the source branch for this pull request (master) is present in the target repository. If you eventually integrate this pull request then the branch master in your personal fork will diverge once you sync your personal fork with the upstream repository.

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$ git push -f origin master

Then proceed to create a new pull request with NEW-BRANCH-NAME as the source branch and close this one.