See JENKINS-42228.

This is a workaround for TypeError by handling undefined object on the Jenkins system configuration page.

The error still occurs in Jenkins 2.250.
Manage Jenkins > Configure Ststem

behavior.js:119 TypeError: Cannot read property 'firstChild' of undefined
    at registerValidator (hudson-behavior.js:511)
    at Array.forEach (<anonymous>)
    at behavior.js:116
    at Array.forEach (<anonymous>)
    at behavior.js:111
    at Array.forEach (<anonymous>)
    at Object.applySubtree (behavior.js:93)
    at select.js:280

The permanent solution is probably to fix the HTML structure or something.
I tried to investigate the root cause, but it seems to be a timing problem and a little bit hard to figure out.
Even so, this workaround would not take a change for the worse.

Here is the error details for further investigation.

Proposed changelog entries

• Entry 1: Prevent JavaScript error when registering validators in some cases

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).