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

## Error

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

## Stack trace

```
/card/card-injection.js:1
/card/card-injection.js:1 in f.get-new-images-list
/card/card-injection.js:1
/card/card-injection.js:1
../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/146894871/)
- **Culprit**: ?(card/card-injection)
- **First seen:** Sep 14, 2026

Start with /card/card-injection.js and the linked Sentry issue, then trace the get-new-images-list path to identify why the postMessage receiver is null. Reproduce or inspect the reported error and confirm that the TypeError no longer occurs in the affected card flow.

Written by the indexing model from the issue text.