[SITES-35203] fix(host-react): prevent GuestUIFrame crash when guest is undefined

[valeriat-ens]

Jira

SITES-35203

Summary

• Guard against a missing host or guest before resolving the frame URL and attaching the UI, so an unavailable guest renders null instead of throwing.
• Hooks now run unconditionally to satisfy the rules of hooks.
• Add unit tests for GuestUIFrame and a jest moduleNameMapper to strip the .js extension from relative imports.

Root cause

Reproduction: a local dummy guest extension whose error handler calls guestConnection.host.modal.close() from within a catch block (per the example in the ticket):

const handleClick = React.useCallback(async () => {
  if (!guestConnection) {
    throw new Error("Required data missing");
  }
  setLoading(true);
  try {
    await new Promise((resolve, reject) =>
      setTimeout(() => reject(new Error("Crash!")), 0)
    );
  } catch (error) {
    console.error("Error:", error);
    guestConnection.host.modal.close();
  }
}, [guestConnection]);

Symptom: calling host.modal.close() tears down the modal and removes the guest. On the next host render, GuestUIFrame is evaluated while the guest no longer exists, and the host UI crashes instead of unmounting cleanly.

Analysis: the failure was not in the extension. In GuestUIFrame, host.guests.get(guestId) can return undefined (guest already removed after the modal closed). The component then ran unconditionally:

• new URL(src, guest.url.href) — throws when guest is undefined.
• useEffect dependencies and bodies keyed on guest.id — dereference undefined.

Because the early return null only covered a missing host (not a missing guest), an undefined guest threw during render and crashed the subtree instead of degrading gracefully.

Fix: resolve the guest defensively (host ? host.guests.get(guestId) : undefined), key the effects on an optional guest?.id, gate effect bodies on guest being present, and return null when either host or guest is missing — after the hooks run, to satisfy the rules of hooks.

Test plan

• npm run test -w packages/uix-host-react (39 passed, incl. 7 new GuestUIFrame tests)
• Verify an undefined/unavailable guest renders nothing instead of crashing the host UI.

[fe-lix-]

@valeriat-ens I have added some E2E test.