Guard against unmounted components when accessing public instances on Fabric (facebook#27687)

## 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">

Author: rubennorte

packages/react-native-renderer/src/ReactFiberConfigFabric.js

@@ -278,13 +278,21 @@ function getPublicTextInstance(
 export function getPublicInstanceFromInternalInstanceHandle(
   internalInstanceHandle: InternalInstanceHandle,
 ): null | PublicInstance | PublicTextInstance {
+  const instance = internalInstanceHandle.stateNode;
+
+  // React resets all the fields in the fiber when the component is unmounted
+  // to prevent memory leaks.
+  if (instance == null) {
+    return null;
+  }
+
   if (internalInstanceHandle.tag === HostText) {
-    const textInstance: TextInstance = internalInstanceHandle.stateNode;
+    const textInstance: TextInstance = instance;
     return getPublicTextInstance(textInstance, internalInstanceHandle);
   }
 
-  const instance: Instance = internalInstanceHandle.stateNode;
-  return getPublicInstance(instance);
+  const elementInstance: Instance = internalInstanceHandle.stateNode;
+  return getPublicInstance(elementInstance);
 }
 
 export function prepareForCommit(containerInfo: Container): null | Object {

packages/react-native-renderer/src/ReactNativeTypes.js

@@ -245,7 +245,7 @@ export type ReactFabricType = {
   ): ?Node,
   getPublicInstanceFromInternalInstanceHandle(
     internalInstanceHandle: InternalInstanceHandle,
-  ): PublicInstance | PublicTextInstance,
+  ): PublicInstance | PublicTextInstance | null,
   ...
 };
 

packages/react-native-renderer/src/__tests__/ReactFabric-test.internal.js

@@ -1112,6 +1112,16 @@ describe('ReactFabric', () => {
         internalInstanceHandle,
       );
     expect(publicInstance).toBe(viewRef);
+
+    await act(() => {
+      ReactFabric.render(null, 1);
+    });
+
+    const publicInstanceAfterUnmount =
+      ReactFabric.getPublicInstanceFromInternalInstanceHandle(
+        internalInstanceHandle,
+      );
+    expect(publicInstanceAfterUnmount).toBe(null);
   });
 
   it('getPublicInstanceFromInternalInstanceHandle should provide public instances for HostText', async () => {
@@ -1153,5 +1163,16 @@ describe('ReactFabric', () => {
       ReactNativePrivateInterface.createPublicTextInstance.mock.results[0]
         .value;
     expect(publicInstance).toBe(expectedPublicInstance);
+
+    await act(() => {
+      ReactFabric.render(null, 1);
+    });
+
+    const publicInstanceAfterUnmount =
+      ReactFabric.getPublicInstanceFromInternalInstanceHandle(
+        internalInstanceHandle,
+      );
+
+    expect(publicInstanceAfterUnmount).toBe(null);
   });
 });