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

## Error

```
Cannot read properties of null (reading 'map')
```

## Stack trace

```
../app/javascript/components/common/TrackSlugsMultiselector.tsx:11 in TrackSlugsMultiselector
Suspense
../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: Cannot read properties of null (reading 'map')](https://thalamus-ai.sentry.io/issues/112093932/)
- **Culprit**: formatTags(app/javascript/components/common/TrackSlugsMultiselector)
- **First seen:** Apr 15, 2026

Start with app/javascript/components/common/TrackSlugsMultiselector.tsx at line 11 and inspect the formatTags path that receives the null value. Use the linked Sentry report to understand the runtime context and verify that the component no longer throws when this case occurs.

Written by the indexing model from the issue text.