Provide types for all react refs

[robertbrignull]

Everywhere we create or consume a React ref, give it a proper type instead of using Ref<any>.

To make this work I switched to passing null as the initial value instead of leaving it blank. I believe this doesn't change the behaviour of the components in any way.

Leaving the initial value empty was leading to a computed type of Thing | undefined and that was causing problems when trying to pass it to ref={ref} on an element. The ref type already expects null to be a possible value, so using that instead of undefined makes everything happy.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[koesie10]

Thanks, LGTM!

Leaving the initial value empty was leading to a computed type of Thing | undefined and that was causing problems when trying to pass it to ref={ref} on an element. The ref type already expects null to be a possible value, so using that instead of undefined makes everything happy.

It seems like the behavior of passing in null is slightly different than specifying the type as T | null since the first doesn't allow you to directly modify the ref. However, we don't need that, so this seems like it works for better for us.