fix(link-dialog): don't crash when the selection rectangle is unavailable

[simplydt]

Summary

Fixes #753 — clicking the Create Link toolbar button while an image is selected crashes the whole editor with TypeError: Cannot read properties of null (reading 'top') (reproducible on the official demo).

Root cause

getSelectionRectangle() returns null when the native DOM selection is not inside the editor root element — which is the case when an image (decorator node) holds the selection, since the Lexical selection and the native selection diverge there (as noted in #753). openLinkEditDialog$ force-asserts the result as non-null:

const rectangle = getSelectionRectangle(editor)!

so a state with rectangle: null reaches LinkDialog, which reads theRect.top and throws, unmounting the editor tree.

Fix

Remove the non-null assertion and fall back to the editor root element's bounding rectangle, so the dialog still opens (anchored to the editor) instead of crashing. This follows the graceful-degradation behavior requested in #753, and mirrors the null handling that the link preview path already does a few lines above (which bails to inactive when getSelectionRectangle returns null).

Testing

• npm run typecheck — clean
• npx eslint src/plugins/link-dialog/index.ts — clean
• npm run test:once — 79 passed, 1 skipped
• Manually verified the equivalent change against 3.52.1: insert an image → click Create Link → dialog opens anchored to the editor, no crash

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