## Summary
On page load, the playground very briefly flickers a JSON parse error.
Due to our use of `useDeferredValue`, we attempt to parse the empty JSON
string settings, since after `const initialized = ruffVersion != null;`
returns true, we get one render with the stale deferred value.
This PR refactors the state, such that we start by storing `null` for
the `Source`, and use the `Source` itself to determine initialization
status.
## Test Plan
Set a breakpoint in the `catch` path in `Editor`; verified that it no
longer triggers on load (but did on `main`).
a1da9da0ef