コンテストが終了した後に、もう一度時間を変更して再開するとランキングが出力されなくなる

[getty104]

コンテストが終了した後に、時間変更するとランキングが出力されなくなる
エラー内容

2018-05-19T03:46:39.930946+00:00 app[web.1]: I, [2018-05-19T12:46:39.930862 #11]  INFO -- : [560d5b1a-0439-4f0e-adc6-52b8e735fa7b] Completed 500 Internal Server Error in 49ms (ActiveRecord: 23.4ms)
2018-05-19T03:46:39.931837+00:00 app[web.1]: F, [2018-05-19T12:46:39.931777 #11] FATAL -- : [560d5b1a-0439-4f0e-adc6-52b8e735fa7b]   
2018-05-19T03:46:39.931975+00:00 app[web.1]: F, [2018-05-19T12:46:39.931899 #11] FATAL -- : [560d5b1a-0439-4f0e-adc6-52b8e735fa7b] NoMethodError (undefined method `[]' for nil:NilClass):
2018-05-19T03:46:39.932108+00:00 app[web.1]: F, [2018-05-19T12:46:39.932035 #11] FATAL -- : [560d5b1a-0439-4f0e-adc6-52b8e735fa7b]   
2018-05-19T03:46:39.932218+00:00 app[web.1]: F, [2018-05-19T12:46:39.932163 #11] FATAL -- : [560d5b1a-0439-4f0e-adc6-52b8e735fa7b] app/controllers/concerns/api/contests_ranking_json.rb:63:in `block in calculate_ranking_score'
2018-05-19T03:46:39.932221+00:00 app[web.1]: [560d5b1a-0439-4f0e-adc6-52b8e735fa7b] app/controllers/concerns/api/contests_ranking_json.rb:58:in `map'
2018-05-19T03:46:39.932223+00:00 app[web.1]: [560d5b1a-0439-4f0e-adc6-52b8e735fa7b] app/controllers/concerns/api/contests_ranking_json.rb:58:in `calculate_ranking_score'
2018-05-19T03:46:39.932225+00:00 app[web.1]: [560d5b1a-0439-4f0e-adc6-52b8e735fa7b] app/controllers/concerns/api/contests_ranking_json.rb:12:in `json_for_ranking'
2018-05-19T03:46:39.932227+00:00 app[web.1]: [560d5b1a-0439-4f0e-adc6-52b8e735fa7b] app/controllers/api/contests_controller.rb:38:in `ranking'
2018-05-19T03:47:02.577346+00:00 app[web.1]: I, [2018-05-19T12:47:02.577223 #11]  INFO -- : [17a163c4-5731-4345-84d4-c53d49604164] Started GET "/api/contests/19/ranking" for 126.247.208.2 at 2018-05-19 12:47:02 +0900
2018-05-19T03:47:02.578284+00:00 app[web.1]: I, [2018-05-19T12:47:02.578217 #11]  INFO -- : [17a163c4-5731-4345-84d4-c53d49604164] Processing by Api::ContestsController#ranking as */*