Compact children when updating parents to children. (#2072)

This PR fixes (or rather buries) a bug that could occur when the parents
to children array includes shapes that no longer exist.

### Change Type

- [x] `patch` — Bug fix

packages/editor/src/lib/editor/derivations/parentsToChildren.ts

@@ -1,6 +1,7 @@
 import { computed, isUninitialized, RESET_VALUE } from '@tldraw/state'
 import { RecordsDiff } from '@tldraw/store'
 import { isShape, TLParentId, TLRecord, TLShape, TLShapeId, TLStore } from '@tldraw/tlschema'
+import { compact } from '@tldraw/utils'
 import { sortByIndex } from '../../utils/reordering/reordering'
 
 type Parents2Children = Record<TLParentId, TLShapeId[]>
@@ -102,7 +103,8 @@ export const parentsToChildren = (store: TLStore) => {
 
 			// Sort the arrays that have been marked for sorting
 			for (const arr of toSort) {
-				const shapesInArr = arr.map((id) => store.get(id)!)
+				// It's possible that some of the shapes may be deleted. But in which case would this be so?
+				const shapesInArr = compact(arr.map((id) => store.get(id)))
 				shapesInArr.sort(sortByIndex)
 				arr.splice(0, arr.length, ...shapesInArr.map((shape) => shape.id))
 			}