Guard against unmounted components when accessing public instances on…

… Fabric (facebook#27687) (facebook#41452)

Summary:

## Summary

This fixes an error in `getPublicInstanceFromInstanceHandle` where we
throw an error when trying to access the public instance from the fiber
of an unmounted component. This shouldn't throw but return `null`
instead.

## How did you test this change?

Updated unit tests.
Before:
<img width="969" alt="Screenshot 2023-11-10 at 15 26 14"
src="https://github.com/facebook/react/assets/117921/ea161616-2775-4fab-8d74-da4bef48d09a">

After:
<img width="1148" alt="Screenshot 2023-11-10 at 15 28 37"
src="https://github.com/facebook/react/assets/117921/db18b918-b6b6-4925-9cfc-3b4b2f3ab92d">

DiffTrain build for commit facebook/react@6b3834a.

Test Plan: Sandcastle tests

Reviewed By: tyao1

Differential Revision: D51203276

Pulled By: tyao1

packages/react-native/Libraries/Renderer/shims/ReactNativeTypes.js

@@ -7,7 +7,7 @@
  * @noformat
  * @flow strict
  * @nolint
- * @generated SignedSource<<652b117c94307244bcf5e4af18928903>>
+ * @generated SignedSource<<1836a1b6639552dce12199ef2c85f63d>>
  */
 
 import type {ElementRef, ElementType, Element, AbstractComponent} from 'react';
@@ -247,7 +247,7 @@ export type ReactFabricType = {
   ): ?Node,
   getPublicInstanceFromInternalInstanceHandle(
     internalInstanceHandle: InternalInstanceHandle,
-  ): PublicInstance | PublicTextInstance,
+  ): PublicInstance | PublicTextInstance | null,
   ...
 };