Fix iframe crash (#4219)

* fixes #4170

* add smoke test for iframe example

* add changeset

* update changeset wording

packages/slate-react/src/plugin/react-editor.ts

@@ -106,6 +106,10 @@ export const ReactEditor = {
     const el = ReactEditor.toDOMNode(editor, editor)
     const root = el.getRootNode()
 
+    // The below exception will always be thrown for iframes because the document inside an iframe
+    // does not inherit it's prototype from the parent document, therefore we return early
+    if (el.ownerDocument !== document) return el.ownerDocument
+
     if (!(root instanceof Document || root instanceof ShadowRoot))
       throw new Error(
         `Unable to find DocumentOrShadowRoot for editor element: ${el}`