TypeError: can't access property "3.5", x is undefined

## Error

```
can't access property "3.5", x is undefined
```

## Stack trace

```
../app/javascript/components/Editor.tsx:90 in Editor_default
../node_modules/@sentry/react/src/errorboundary.tsx:101 in new ErrorBoundary
../node_modules/@tanstack/react-query/src/QueryClientProvider.tsx:29 in QueryClientProvider
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: can't access property "3.5", x is undefined](https://thalamus-ai.sentry.io/issues/141462651/)
- **Culprit**: Editor_default(app/javascript/components/Editor)
- **First seen:** Aug 18, 2026

Start at app/javascript/components/Editor.tsx:90 and use the linked Sentry report to understand the TypeError in the exercism-js project. Trace the undefined value and reproduce the failing Editor path; done means the reported error no longer occurs and the relevant behavior remains working.

Written by the indexing model from the issue text.