Fix TypeScript type error in example code (#84041)
Without the `| null`, the following error appears during compilation:
```
TS2322: Type RefObject<HTMLDivElement | null> is not assignable to type RefObject<HTMLDivElement>
Type HTMLDivElement | null is not assignable to type HTMLDivElement
Type null is not assignable to type HTMLDivElement
```
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
