TypeError: null is not an object (evaluating 'o.dom.parentNode')

## Error

```
null is not an object (evaluating 'o.dom.parentNode')
```

## Stack trace

```
../app/javascript/components/misc/CodeMirror.tsx:29 in CodeMirror
../app/javascript/components/common/Tab.tsx:58 in TabPanel
../app/javascript/components/editor/FileEditorCodeMirror.tsx:31 in FileEditorCodeMirror
../app/javascript/components/common/SplitPane.tsx:28 in SplitPane
../app/javascript/components/Editor.tsx:94 in Editor_default
../node_modules/@sentry/react/src/errorboundary.tsx:101 in new ErrorBoundary
../node_modules/@tanstack/react-query/src/QueryClientProvider.tsx:30 in QueryClientProvider
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: null is not an object (evaluating 'o.dom.parentNode')](https://thalamus-ai.sentry.io/issues/135993643/)
- **Culprit**: d(app/javascript/components/misc/CodeMirror)
- **First seen:** Jul 23, 2026

Start at app/javascript/components/misc/CodeMirror.tsx:29, then trace the reported path through Tab.tsx, FileEditorCodeMirror.tsx, SplitPane.tsx, and Editor.tsx. Reproduce the Sentry error for the exercism-js project and determine when the DOM parent is null; done means that scenario no longer raises the reported TypeError.

Written by the indexing model from the issue text.