NoMethodError: undefined method `username' for nil:NilClass

Sentry Issue: GITLABCOM-K6DF

NoMethodError: undefined method `username' for nil:NilClass
  gitlab/hook_data/group_member_builder.rb:40:in `group_member_data'
    user_username: group_member.user.username,
  ee/gitlab/hook_data/group_member_builder.rb:12:in `group_member_data'
    super.tap do |data|
  gitlab/hook_data/group_member_builder.rb:28:in `build'
    group_member_data,
  system_hooks_service.rb:38:in `build_event_data'
    builder_class.new(model).build(event)
  system_hooks_service.rb:5:in `execute_hooks_for'
    data = build_event_data(model, event)
...
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