Model#modifySelection() crash on option+backspace with mixed inline formatting

[niegowski]

📝 Provide detailed reproduction steps (if any)

1. https://ckeditor.com/docs/ckeditor5/latest/examples/builds/classic-editor.html
2. editor.setData( '<strong>fo</strong><i><strong>o</strong>bar</i>' )
3. put caret at the end of line
4. press option+backspace

✔️ Expected result

Word deleted.

❌ Actual result

Uncaught TypeError: Cannot read properties of null (reading 'startOffset')
    at snippet.js:4:320608
    at Hh (snippet.js:4:320639)
    at snippet.js:4:324865
    at Wh.modifySelection (snippet.js:4:324968)
    at Wh.<anonymous> (snippet.js:4:6692)
    at Wh.fire (snippet.js:4:3066)
    at <computed> [as modifySelection] (snippet.js:4:6744)
    at Object.callback (snippet.js:4:417073)
    at Wh._runPendingChanges (snippet.js:4:326815)
    at Wh.enqueueChange (snippet.js:4:322856)

❓ Possible solution

If you have ideas, you can list them here. Otherwise, you can delete this section.

📃 Other details

• Browser: …
• OS: …
• First affected CKEditor version: …
• Installed CKEditor plugins: …

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