Update findDocumentOrShadowRoot to return the root node instead of throwing unnecessarily (#4427)

* Update `findDocumentOrShadowRoot` to return undefined instead of throwing unnecessarily

* Small refactoring to improve the diff for reviewers

* Add changeset for patch

* Update new-trainers-peel.md

* Resolve PR comments

* Revert undefined checks, return window.document and update changeset

* Simplify findDocumentOrShadowRoot based on PR feedback

* Re-run CI

Thanks everyone for your review and thanks @ben10code for your first contribution!

.changeset/new-trainers-peel.md

@@ -0,0 +1,5 @@
+---
+'slate-react': patch
+---
+
+Fix crash when unmounting an editor rendered within a React portal. The issue was arising at unmount time, because `getRootNode` returned the dettached portal node and it is not an instance of `Document` or `ShadowRoot`. As a fix, `getDocumentOrShadowRoot` has been refactored to return a root node instead of throwing. In sum, this patch fixes a regression bug introduced by https://github.com/ianstormtaylor/slate/pull/3749/

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

@@ -106,23 +106,14 @@ 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
+    if (
-    // does not inherit it's prototype from the parent document, therefore we return early
+      (root instanceof Document || root instanceof ShadowRoot) &&
-    if (el.ownerDocument !== document) return el.ownerDocument
+      root.getSelection != null
+    ) {
+      return root
+    }
 
-    if (!(root instanceof Document || root instanceof ShadowRoot))
+    return el.ownerDocument
-      throw new Error(
-        `Unable to find DocumentOrShadowRoot for editor element: ${el}`
-      )
-
-    // COMPAT: Only Chrome implements the DocumentOrShadowRoot mixin for
-    // ShadowRoot; other browsers still implement it on the Document
-    // interface. (2020/08/08)
-    // https://developer.mozilla.org/en-US/docs/Web/API/ShadowRoot#Properties
-    if (root.getSelection === undefined && el.ownerDocument !== null)
-      return el.ownerDocument
-
-    return root
   },
 
   /**