TypeError: undefined is not an object (evaluating 'window.webkit.messageHandlers')

## Error

```
undefined is not an object (evaluating 'window.webkit.messageHandlers')
```

## Stack trace

```
/tracks/java:1 in sendDataToNative
/tracks/java:1 in sendPageHideMessage
/tracks/java:1
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: undefined is not an object (evaluating 'window.webkit.messageHandlers')](https://thalamus-ai.sentry.io/issues/131786954/)
- **Culprit**: sendDataToNative(tracks/java)
- **First seen:** Jul 2, 2026

Start with the Sentry report and the sendDataToNative entry point shown in the /tracks/java stack trace. Trace the sendPageHideMessage path and verify that the reported TypeError no longer occurs when the WebKit message handler is unavailable.

Written by the indexing model from the issue text.