Fixed incorrect value returned as public instance from reconciler

[rubennorte]

Summary

A few methods in ReactFiberReconciler are supposed to return PublicInstance values, but they return the stateNode from the fiber directly. This assumes that the stateNode always matches the public instance (which it does on Web) but that's not the case in React Native, where the public instance is a field in that object.

This hasn't caused issues because everywhere where we use that method in React Native we actually extract the real public instance from this "fake" public instance.

This PR fixes the inconsistency and cleans up some code.

How did you test this change?

Existing tests.

[react-sizebot]

Comparing: d49e0e0...90fff06

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	155.25 kB	155.25 kB	=	48.98 kB	48.98 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	157.24 kB	157.24 kB	=	49.64 kB	49.64 kB
facebook-www/ReactDOM-prod.classic.js	=	532.50 kB	532.50 kB	=	94.85 kB	94.85 kB
facebook-www/ReactDOM-prod.modern.js	=	516.42 kB	516.42 kB	=	92.45 kB	92.45 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 90fff06

[gnoff]

This can return undefined but the places where getPublicInstance are called require an instance or null. I would have thought flow would error on this.

also, is _nativeTag === 0 a special sentinel value? I assume this is actually true and why this logic makes sense but given I'm fairly unfamiliar with the native side I thought I'd point it out in case there was some oversight here

[rubennorte]

This can return undefined but the places where getPublicInstance are called require an instance or null. I would have thought flow would error on this.

I assumed so but apparently getPublicInstance is typed as any :S

also, is _nativeTag === 0 a special sentinel value? I assume this is actually true and why this logic makes sense but given I'm fairly unfamiliar with the native side I thought I'd point it out in case there was some oversight here

It could be, yeah. Flow didn't catch this because from a typing perspective we never expect this branch to happen (it's only if you happen to use Paper and Fabric on the same app and passed a Paper reference to Fabric for some reason). The value is empty, so it doesn't complain. I'll make the code more resilient.

Thanks for the feedback!