Guard against unmounted components when using traversal APIs (facebook#41451)

Summary:

After [this change in React](facebook/react#27687), `ReactFabric.getPublicInstanceFromInternalInstanceHandle` can return `null` if the instance handle is a fiber that was unmounted (before that PR, it would throw an error).

This modifies the DOM traversal API to gracefully handle that case.

Changelog: [internal]

Reviewed By: rshest

Differential Revision: D51210455

Author: rubennorte

packages/react-native/Libraries/DOM/Nodes/ReactNativeElement.js

@@ -90,7 +90,7 @@ export default class ReactNativeElement
           offsetParentInstanceHandle,
         );
         // $FlowExpectedError[incompatible-type] The value returned by `getOffset` is always an instance handle for `ReadOnlyElement`.
-        const offsetParentElement: ReadOnlyElement = offsetParent;
+        const offsetParentElement: ReadOnlyElement | null = offsetParent;
         return offsetParentElement;
       }
     }

packages/react-native/Libraries/DOM/Nodes/ReadOnlyNode.js

@@ -134,7 +134,9 @@ export default class ReadOnlyNode {
       return null;
     }
 
-    return getPublicInstanceFromInternalInstanceHandle(parentInstanceHandle);
+    return (
+      getPublicInstanceFromInternalInstanceHandle(parentInstanceHandle) ?? null
+    );
   }
 
   get previousSibling(): ReadOnlyNode | null {
@@ -322,9 +324,11 @@ export function getChildNodes(
   const childNodeInstanceHandles = nullthrows(
     getFabricUIManager(),
   ).getChildNodes(shadowNode);
-  return childNodeInstanceHandles.map(instanceHandle =>
-    getPublicInstanceFromInternalInstanceHandle(instanceHandle),
-  );
+  return childNodeInstanceHandles
+    .map(instanceHandle =>
+      getPublicInstanceFromInternalInstanceHandle(instanceHandle),
+    )
+    .filter(Boolean);
 }
 
 function getNodeSiblingsAndPosition(
@@ -348,7 +352,7 @@ function getNodeSiblingsAndPosition(
 
 export function getPublicInstanceFromInternalInstanceHandle(
   instanceHandle: InternalInstanceHandle,
-): ReadOnlyNode {
+): ?ReadOnlyNode {
   const mixedPublicInstance =
     ReactFabric.getPublicInstanceFromInternalInstanceHandle(instanceHandle);
   // $FlowExpectedError[incompatible-return] React defines public instances as "mixed" because it can't access the definition from React Native.