sidekiq_middleware.rb

class QuitWhenEmpty
  def initialize(options=nil)
  end

  def call(worker, msg, queue)
    yield
    stop_disposible_worker_if_queue_empty!
  end

  private

  def stop_disposible_worker_if_queue_empty!
    if kill_when_queue_empty?
      my_process.stop! if queue_empty?
    end
  end

  def kill_when_queue_empty?
    ENV["SIDEKIQ_KILL_WHEN_QUEUE_EMPTY"]
  end

  def my_process
    ps = Sidekiq::ProcessSet.new
    ps.detect {|p| Process.pid == p['pid'] }
  end

  def queue_empty?
    Sidekiq::Queue.new.size == 0
  end
end

I don't think you need the initializer