TypeError: can't access property "contentWindow", document.querySelector(...) is null

## Error

```
can't access property "contentWindow", document.querySelector(...) is null
```

## Stack trace

```
/tracks/go/exercises/hello-world/edit:7 in closeModal
/tracks/go/exercises/hello-world/edit:2
/tracks/go/exercises/hello-world/edit:3
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: can't access property "contentWindow", document.querySelector(...) is null](https://thalamus-ai.sentry.io/issues/139688145/)
- **Culprit**: closeModal(tracks/go/exercises/hello-world/edit)
- **First seen:** Aug 9, 2026

Start with the closeModal entry point shown in the /tracks/go/exercises/hello-world/edit stack trace and inspect why document.querySelector(...) is null when the error occurs. Reproduce the Sentry TypeError and verify that closing the modal no longer accesses a missing element.

Written by the indexing model from the issue text.