fix: Make Reflector HMR-proof (#292)

Any time we wire up a useFrame, furiously null-check refs inside it as an HMR (in Next.js) will trigger an unmount but not before a final frame ticks off, crashing us.

src/core/Reflector.tsx

@@ -171,6 +171,7 @@ export function Reflector({
   ])
 
   useFrame(() => {
+    if (!meshRef?.current) return
     meshRef.current.visible = false
     beforeRender()
     gl.setRenderTarget(fbo1)