Describe the bug
During a GET /stats request, we received the error message undefined method “reset_max” for nil (NoMethodError) @thread_pool.reset_max.

    def stats
      stats = @thread_pool&.stats || {}          # considers that @thread_pool might be nil
      stats[:max_threads]    = @max_threads
      stats[:requests_count] = @requests_count
      stats[:reactor_max] = @reactor.reactor_max if @reactor
      reset_max                                  # calls reset_max
      stats
    end

    def reset_max
      @reactor.reactor_max = 0 if @reactor
      @thread_pool.reset_max                     # does not consider @thread_pool to be nil
    end

A quick look at the stack trace revealed that the method Puma::Server.reset_max does not consider @thread_pool to be nil. However, the calling method Puma::Server.stats considers @thread_pool to be nil. Therefore, I assume that reset_max, since it is called from stats, should apply the same consideration with regard to nil values.

Full Stack Trace

NoMethodError: undefined method 'reset_max' for nil (NoMethodError)

      @thread_pool.reset_max
                  ^^^^^^^^^^
    from puma/server.rb:709:in 'reset_max'
    from puma/server.rb:703:in 'stats'
    from puma/single.rb:20:in 'stats'
    from puma/launcher.rb:116:in 'stats'
    from puma/app/status.rb:55:in 'call'
    from puma/request.rb:101:in 'block in Puma::Request#handle_request'
    from puma/thread_pool.rb:346:in 'with_force_shutdown'
    from puma/request.rb:100:in 'handle_request'
    from puma/server.rb:506:in 'process_client'
    from puma/server.rb:265:in 'block in Puma::Server#run'
    from puma/thread_pool.rb:173:in 'block in Puma::ThreadPool#spawn_thread'

To Reproduce
I'm actually not sure how we managed to cause it. Maybe just bad timing during shutdown? (wild guess)

Desktop (please complete the following information):

• OS: Linux
• Ruby: 3.4.2
• Puma: 7.0.4