TypeError: object null is not iterable (cannot read property Symbol(Symbol.iterator))

[sentry]

Sentry Issue: REACT-6

TypeError: object null is not iterable (cannot read property Symbol(Symbol.iterator))
  at checkContest (routes.js:44:41)
  at c (routes.js:88:31)
  at Module.1125 (index.js:29:10)
  at i (/contests/JCJ20:1:1201)
  at r (/contests/JCJ20:1:1070)
...
(14 additional frame(s) were not displayed)