Bug: `TypeError: Cannot read properties of null (reading 'style')` when component unmounts before script loads

[KieranP]

(disclaimer: issue description and reproducible test case produced by Claude Open 4.7, but verified by a human for accuracy)

Summary

If an Editor instance unmounts before tinymce.min.js finishes loading, the script's onload later runs an orphaned init() callback and throws TypeError: Cannot read properties of null (reading 'style') at Editor.svelte line 182 (element!.style.visibility = '').

Minimal reproduction

1. Throttle the network in DevTools to "Slow 3G" (or anything that makes the TinyMCE script take >100ms).
2. Mount the component below — it unmounts itself after 50ms.
3. Observe the uncaught TypeError when the script eventually loads.

<script lang="ts">
  import Editor from '@tinymce/tinymce-svelte';

  let mounted = $state(true);

  // Unmount before the TinyMCE script can finish loading.
  // Any value smaller than the script's load time will reproduce.
  $effect(() => {
    const id = setTimeout(() => { mounted = false; }, 50);
    return () => clearTimeout(id);
  });
</script>

{#if mounted}
  <Editor
    apiKey="no-api-key"
    channel="8"
  />
{/if}

<!--
  Expected: clean unmount, no console errors.
  Actual: when the TinyMCE script finishes loading, an uncaught
          TypeError: Cannot read properties of null (reading 'style')
          is thrown from the orphan init() callback.
-->

In a real app, the trigger is more organic: a user navigates to a page containing a <Editor>, then navigates away before the network has finished delivering tinymce.min.js. We observed this in production from users on slower devices/connections.

Suggested fix

Set a destroyed flag in onDestroy and short-circuit the queued init callback if set. Ideally also splice the listener out of state.listeners to avoid unbounded growth in long-lived SPAs.