NoMethodError: undefined method `sort_by_attribute' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1561789/?referrer=gitlab_plugin

NoMethodError: undefined method `sort_by_attribute' for nil:NilClass
  issuable_finder.rb:342:in `sort'
    params[:sort] ? items.sort_by_attribute(params[:sort], excluded_labels: params.label_names) : items.reorder(id: :desc)
  gitlab/metrics/instrumentation.rb:161:in `block in sort'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `sort'
    .measure { super }
  issuable_finder.rb:111:in `execute'
    items = sort(items)
...
(146 additional frame(s) were not displayed)

NoMethodError: undefined method `sort_by_attribute' for nil:NilClass