Summary: When calling from Kotlin codes, making nullability clear is safer. This PR adds `Nullable` annotation to `getInstanceIfInitialized`. ## Changelog Add `Nullable` annotation to `getInstanceIfInitialized` Pull Request resolved: https://github.com/facebook/flipper/pull/770 Reviewed By: mweststrate Differential Revision: D19719980 Pulled By: passy fbshipit-source-id: 195b396a8fd75a15247da7244215aca9ed35886c