Fix crash in `fixMountingCoordinatorReportedPendingTransactionsOnAndroid` feature flag (#50088)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50088

Changelog: [internal]

This fixes a crash when using the `fixMountingCoordinatorReportedPendingTransactionsOnAndroid` feature flag. The problem was that we were accessing the surface handler registry safely, but there were cases where invalid surface handler references were being kept in the registry.

Reviewed By: javache

Differential Revision: D71387111

fbshipit-source-id: 643fcdf10c4c5751d77e276efb37795b7af726bb