NoMethodError: undefined method `username' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/740171/

NoMethodError: undefined method `username' for nil:NilClass
  members/create_service.rb:26:in `block in execute'
    errors << "#{member.user.username}: #{member.errors.full_messages.to_sentence}"
  members/create_service.rb:24:in `each'
    members.each do |member|
  members/create_service.rb:24:in `execute'
    members.each do |member|
  gitlab/metrics/instrumentation.rb:161:in `block in execute'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
...
(114 additional frame(s) were not displayed)

NoMethodError: undefined method `username' for nil:NilClass