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

## Error

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

## Stack trace

```
/asset/asset.js:2 in A
/asset/asset.js:2 in Object.getItem
/asset/asset.js:2 in F
/asset/asset.js:2 in U
/asset/asset.js:2 in Object.initCallback
/asset/asset.js:2 in e
../node_modules/@sentry/browser/src/helpers.ts:117 in sentryWrapped
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: Cannot read properties of null (reading 'postMessage')](https://thalamus-ai.sentry.io/issues/144940638/)
- **Culprit**: A(asset/asset)
- **First seen:** Sep 4, 2026

Start with the linked Sentry report and the stack trace for /asset/asset.js, especially the getItem and initCallback frames. Trace those frames back to the project source and determine when the null postMessage access occurs. Done means the reported TypeError no longer occurs in the affected initialization path, with a regression test or reproduction check where available.

Written by the indexing model from the issue text.