The TypeScript example does feel a little weird. I've used this library successfully with React + TypeScript with the following pattern:

  const mesh = useRef<THREE.Mesh>(null); // mesh has type RefObject<THREE.Mesh | null>
  ...
  useFrame((): void => {
    if (mesh.current != null) {
      mesh.current.rotation.x += 0.01;
    }
  });
  ...
  return <mesh ref={mesh} ... />

As others have mentioned, the ref will typically attach by the time a `useEffect` hook is called, but for safety it's nice to have the null check.