NoMethodError: undefined method `sort_by_attribute' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1177326/

NoMethodError: undefined method `sort_by_attribute' for nil:NilClass
  group_members_finder.rb:50:in `filter_members'
    members = members.sort_by_attribute(params[:sort]) if params[:sort].present?
  gitlab/metrics/instrumentation.rb:161:in `block in filter_members'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `filter_members'
    .measure { super }
  group_members_finder.rb:40:in `execute'
    filter_members(members, params)
...
(136 additional frame(s) were not displayed)

NoMethodError: undefined method `sort_by_attribute' for nil:NilClass