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
../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/144940652/)
- **Culprit**: A(asset/asset)
- **First seen:** Sep 4, 2026

Start with the Sentry report and the stack trace entry points in /asset/asset.js, especially Object.getItem. Determine the browser state that leaves the postMessage target null and reproduce the failure before changing anything. Done means the reported TypeError no longer occurs in the affected flow.

Written by the indexing model from the issue text.