Updates to the session store when rendering _error.svelte causes a TypeError

[AaronDDM]

Describe the bug
If you are on a 404 page handled by Sapper and your code (e.g. in the _layout.svelte file) updates the session store, you'll notice a TypeError in your console logs.

Logs

Uncaught (in promise) TypeError: Cannot destructure property 'route' of 'dest' as it is undefined.

To Reproduce
https://github.com/AaronDDM/sapper-store-update-bug/blob/main/src/routes/_layout.svelte

1. Check out the above repo
2. Start your server via npm run dev
3. Visit a URL that does not exist (e.g. http://localhost:3000/ARouteThatDoesNotExist)
4. Open your browser console logs
5. Refresh the page

Expected behavior
No type error.

Stacktraces
If you have a stack trace to include, we recommend putting inside a <details> block for the sake of the thread's readability:

Stack trace

	(anonymous)	@	app.mjs:455
	(anonymous)	@	app.mjs:28
	__awaiter	@	app.mjs:24
	hydrate_target	@	app.mjs:454
	(anonymous)	@	app.mjs:334
	(anonymous)	@	app.mjs:28
	__awaiter	@	app.mjs:24
	(anonymous)	@	app.mjs:327
	set	@	index.mjs:35
	update	@	index.mjs:43
	(anonymous)	@	_layout.svelte:11
	Promise.then (async)
	step	@	app.mjs:27
	(anonymous)	@	app.mjs:28
	__awaiter	@	app.mjs:24
	(anonymous)	@	app.mjs:327
	set	@	index.mjs:35
	update	@	index.mjs:43
	(anonymous)	@	_layout.svelte:11
	setTimeout (async)
	instance$2	@	_layout.svelte:10
	init$1	@	index.mjs:1474
	Layout	@	_layout.svelte:17
	create_fragment	@	App.svelte:20
	init$1	@	index.mjs:1489
	App	@	App.svelte:17
	(anonymous)	@	app.mjs:424
	fulfilled	@	app.mjs:25
	Promise.then (async)
	step	@	app.mjs:27
	(anonymous)	@	app.mjs:28
	__awaiter	@	app.mjs:24
	render	@	app.mjs:408
	handle_error	@	app.mjs:382
	(anonymous)	@	app.mjs:354
	Promise.then (async)
	start$1	@	app.mjs:353
	(anonymous)	@	client.js:3