[plasmicpkgs] Fix Embed HTML crash when a <script> is detached mid-rehydration

[jlubawy]

Problem

Embed (Embed HTML) re-executes embedded <script> tags in an async loop, awaiting each external (src) script's load event before continuing. Every iteration asserts:

ensure(oldScript.parentNode).replaceChild(newScript, oldScript);

If a re-render occurs while an earlier script's load is being awaited, React re-applies dangerouslySetInnerHTML and detaches the original <script> nodes that were captured at the start of the loop. On the next iteration oldScript.parentNode is null, so ensure() throws:

Value must not be undefined or null

Because the throw happens in a post-await microtask it becomes an unhandled promise rejection, which is reported as an "application error" even though the page content already rendered. It reproduces reliably on a page whose embed loads an external script (e.g. an analytics/tag loader such as CallRail): the first src script's awaited load opens the window, and the next iteration throws.

While here, I also noticed the cleanup function was returned from the async IIFE rather than from the useEffect callback, so it was never registered — cleanup never became true and the existing post-await guard was dead code.

Fix

• Skip <script> nodes whose parentNode has become null (detached by a re-render) instead of asserting on them.
• Bail out of the loop when the effect cleanup has run (unmount / dependency change).
• Actually register the cleanup by returning it from useEffect (not the async IIFE), so the post-await guard works as intended.
• Capture rootElt.current once and guard it, dropping the now-unused ensure.

No behavior change on the happy path — the same scripts are rehydrated in the same order; the loop now just tolerates a tree that React swapped out from under it instead of throwing.

Notes

Verified against a live site that embeds an external script: without the fix the page throws the assertion as an unhandled rejection on load; with the fix the rehydration completes (or cleanly stops when the DOM is replaced) and no error is thrown.

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