TypeError: undefined is not an object (evaluating 'x["3.5"]')

## Error

```
undefined is not an object (evaluating 'x["3.5"]')
```

## Stack trace

```
../app/javascript/components/Editor.tsx:94 in Editor_default
../node_modules/@sentry/react/src/errorboundary.tsx:101 in new ErrorBoundary
../node_modules/@tanstack/react-query/src/QueryClientProvider.tsx:30 in QueryClientProvider
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: undefined is not an object (evaluating 'x["3.5"]')](https://thalamus-ai.sentry.io/issues/141462775/)
- **Culprit**: Editor_default(app/javascript/components/Editor)
- **First seen:** Aug 18, 2026

Start at app/javascript/components/Editor.tsx:94 and compare the failing access with the Sentry error and stack trace. Trace which value is undefined for the "3.5" lookup, then reproduce the failure in the editor flow and verify that the error no longer occurs; the payload does not name a test file.

Written by the indexing model from the issue text.