fix: don't throw from remote form validate() if the form unmounts

[kdelay]

closes #16696

validate() checks element before await tick(), but the attachment cleanup sets element = null when the <form> unmounts. If the form is removed while validate() is waiting for that tick — e.g. an {#if} flips right after a fast submission — the resumed call throws:

TypeError: Cannot read properties of null (reading 'querySelector')

The fix re-checks element after the tick and bails out if the form is gone, which matches what the pre-tick check already intends.

Tests

Added a case to the async test app's existing /remote/form/validate route: a form inside an {#if} block, and a button that starts validate() and unmounts the form in the same handler, catching whatever the promise rejects with. The new test fails on version-3 (TypeError: Cannot read properties of null (reading 'querySelector')) and passes with this change.

Rebased onto version-3 at a115a7b and re-verified locally on Node 24.18.0:

• pnpm lint: 0 errors (one pre-existing warning in src/runtime/server/page/index.js, untouched here).
• pnpm test:unit: 895 passed, 11 failed. The 11 failures are in kit-prerendering-paths-base and kit-prerendering-options and reproduce identically on unpatched version-3, so they are not from this change.
• async test app, pnpm test:dev: 193 passed, 103 skipped, 0 failed. The new test also passes in chromium-build and chromium-build-no-js.
• pnpm check fails on unpatched version-3 as well, with 24 TS2300: Duplicate identifier errors from src/types/ambient.d.ts. This branch changes no types, so I could not get a clean baseline for that step.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from ad40934:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/ad409342628607eb1466a1c2213e3d099ba447e0

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16720

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: ad40934

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@sveltejs/kit	Patch

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[kdelay]

A note on the red lint-all here: it reproduces on the base commit, so it does not come from this PR.

This branch is based on a115a7beb, which is the current version-3 head. The upstream push run for that same commit fails lint-all as well: https://github.com/sveltejs/kit/actions/runs/31442413468/job/93629686461 (failed 2026-08-10 23:32 UTC). I compared the tsc diagnostics from that run with the ones from this PR's run (https://github.com/sveltejs/kit/actions/runs/31455435935/job/93668121269): both report the same 87 errors with an identical file/line/error-code set, mostly TS2339: Property '_' does not exist on type 'SSRManifest' and the TS2300/TS2451 duplicate identifiers between src/types/ambient.d.ts and types/index.d.ts. The last green version-3 run was 0b3e2b386.

The diff here is limited to packages/kit/src/runtime/client/remote-functions/form.svelte.js and the async test app, and it does not touch any type declarations. Every other job on this run is green, including the svelte-async dev and build suites that cover the new test. I am happy to rebase once version-3 is green again.