Prevent from crashing with null cannot be cast to non-null type kotlin.Any

[lukmccall]

Summary

It's possible for the getInitialWindowMetrics method to return null, which can cause crashes if we force cast it to Any. I faced issues with reloading the app, resulting in errors:

ERROR  Error: Exception in HostObject::get for prop 'RNCSafeAreaContext': java.lang.NullPointerException: null cannot be cast to non-null type kotlin.Any, js engine: hermes

I'm unsure of the expected behavior when getInitialWindowMetrics returns null, but the current exception doesn't provide enough information.

Test Plan

• Tested in expo app ✅

[janicduplessis]

Looks good, thanks!