`useRef(null as T|null)` now returns a `{ current: T|null }` instead of
`{ current: T }` as it did before. ie. it no longer drops the the null.
This makes sense but conflicted with a pattern we used in many places to
create a non-null ref: `useRef(/** @type {T|null} */ (null))`.

Resolve this by changing all non-nullable refs, for elements which are
set after the initial render, to cast the `useRef` result instead of the
init value.

```
const nonNullRef = /** @type {{ current: T }} */ (useRef());
```