IntersectionObserver: Only reset modern io, if it exists (#51510)

Summary:
Pull Request resolved: #51510

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D75077634

fbshipit-source-id: d648a5e8c1de8efd44f4b1d2c44932bfebe0fbb3

Author: RSNara

packages/react-native/src/private/webapis/intersectionobserver/internals/IntersectionObserverManager.js

@@ -92,7 +92,10 @@ let modernNativeIntersectionObserver =
           unobserve: NativeIntersectionObserver.unobserveV2,
         };
 
-if (!ReactNativeFeatureFlags.utilizeTokensInIntersectionObserver()) {
+if (
+  modernNativeIntersectionObserver &&
+  !ReactNativeFeatureFlags.utilizeTokensInIntersectionObserver()
+) {
   modernNativeIntersectionObserver = null;
 }