TypeError: Cannot read properties of null (reading 'length')

Sentry Issue: [TAF-FORM-PRODUCTION-AM](https://sentry.io/organizations/speee/issues/3602121687/?referrer=github_integration)

```
TypeError: Cannot read properties of null (reading 'length')
  at validator (./src/components/UserNameAndAge/util.ts:11:16)
  at h (./src/components/UserNameAndAge/NameKanaInputBox.tsx:28:16)
...
(4 additional frame(s) were not displayed)
```