Error 500 loading a merge request: ActionView::Template::Error: undefined method `[]=' for nil:NilClass

https://sentry.gitlap.com/gitlab/gitlabcom/issues/115504/

NoMethodError: undefined method `[]=' for nil:NilClass
  app/models/diff_discussion.rb:27:in `block in merge_request_version_params'
    params[:commit_id] = commit_id if on_merge_request_commit?
  app/models/diff_discussion.rb:26:in `tap'
    version_params.tap do |params|
  app/models/diff_discussion.rb:26:in `merge_request_version_params'
    version_params.tap do |params|
  app/helpers/notes_helper.rb:83:in `discussion_path'
    version_params = discussion.merge_request_version_params
  app/views/discussions/_discussion.html.haml:23:in `_app_views_discussions__discussion_html_haml___2848104779732742178_70244662970240'
    - url = discussion_path(discussion)
...
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ActionView::Template::Error: undefined method `[]=' for nil:NilClass

ActionView::Template::Error: undefined method `[]=' for nil:NilClass