Add missing null checks to OffscreenInstance code

[acdlite]

stateNode is any-typed, so when reading from stateNode we should always cast it to the specific type for that type of work. I noticed a place in the commit phase where OffscreenInstance wasn't being cast. When I added the type assertion, it exposed some type errors where nullable values were being accessed without first being refined.

I added the required null checks without verifying the logic of the existing code. If the existing logic was correct, then the extra null checks won't have any affect on the behavior, because all they do is refine from a nullable type to a non-nullable type in places where the type was assumed to already be non-nullable. But the result looks a bit fishy to me, so I also left behind some TODOs to follow up and verify it's correct.

[sizebot]

Comparing: 4cd788a...6d1e570

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	=	131.79 kB	131.79 kB	=	42.40 kB	42.40 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	137.06 kB	137.06 kB	=	44.00 kB	44.00 kB
facebook-www/ReactDOM-prod.classic.js	+0.05%	457.17 kB	457.38 kB	+0.02%	83.22 kB	83.24 kB
facebook-www/ReactDOM-prod.modern.js	+0.05%	442.41 kB	442.62 kB	+0.02%	80.94 kB	80.96 kB
facebook-www/ReactDOMForked-prod.classic.js	+0.05%	457.94 kB	458.15 kB	+0.02%	83.33 kB	83.35 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 6d1e570

[lunaruan]

PrevTransitions can't be null because we set it above to a new set if it is

if (transitions !== null && prevTransitions === null) {
    instance.transitions = prevTransitions = new Set();
}

[lunaruan]

instance.transitions also can't be null because we set it above if it is