Placeholder: destroying the editor throws NotFoundError when its <style> was already removed from the head #3144

What's broken?

The Placeholder extension appends an empty <style> element to view.root.head for every editor (its rules go in through sheet.insertRule), and its plugin view removes it on destroy with removeChild:

https://github.com/TypeCellOS/BlockNote/blob/main/packages/core/src/extensions/Placeholder/Placeholder.ts#L91-L97

destroy: () => {
  if (view.root instanceof window.ShadowRoot) {
    view.root.removeChild(styleEl);
  } else {
    view.root.head.removeChild(styleEl);
  }
},

If anything removed that <style> first, removeChild throws:

NotFoundError: Failed to execute 'removeChild' on 'Node': The node to be removed is not a child of this node.

Because the throw happens inside ProseMirror's EditorView.destroyPluginViews, it also aborts the rest of that editor's teardown: the plugin views still on the stack and the doc view are never destroyed.

We hit it with Hotwire Turbo. On a page visit, Turbo merges the new <head> into the current one and drops all but the first of elements with identical outerHTML. Every Placeholder <style> is <style></style>, so with several editors on a page — in our case the comment editors of the threads sidebar — all but one lose their style before the editors unmount, and each of those teardowns throws. Stack from production (React 19.2.4):

NotFoundError: Failed to execute 'removeChild' on 'Node': The node to be removed is not a child of this node.
  at Object.destroy (Placeholder)
  at EditorView.destroyPluginViews
  at EditorView.destroy
  at Editor.unmount
  at BlockNoteEditor.unmount
  at safelyDetachRef
  at commitDeletionEffectsOnFiber

What did you expect to happen?

Destroying the editor should not throw when its <style> is already gone. styleEl.remove() does the same thing and is a no-op when the element has no parent, so it covers both branches:

destroy: () => {
  styleEl.remove();
},

Steps to reproduce

No Turbo needed:

1. Mount an editor (e.g. the minimal example).
2. Remove its Placeholder style, the way a head-merging router would:

   [...document.head.querySelectorAll("style")]
     .filter((s) => /^\.placeholder-selector-/.test(s.sheet?.cssRules[0]?.selectorText))
     .forEach((s) => s.remove());

3. Destroy the editor (unmount the component, or editor._tiptapEditor.destroy()).
4. NotFoundError is thrown from the Placeholder plugin view's destroy.

BlockNote version

0.53.0 (the code is unchanged on main / 0.55.0)

Environment

Chrome 151 and Electron (Cypress), React 19.2.4, Hotwire Turbo 8.0.23

Additional context

Our workaround, until this is fixed: a MutationObserver on document.head gives each Placeholder <style> a distinct attribute, so Turbo no longer treats them as duplicates — Grupo-AFAL/bali-view-components#1224. It works, but it only covers one way of losing the element; remove() would cover all of them.