When the ref callback argument is NULL in react?

Question

In some component when I use ref callback to receive component's ref, the argument of callback is null. When and in what situations the argument will be null?

Answer 1

React will call the ref callback with the DOM element when the component mounts, and call it with null when it unmounts.

https://reactjs.org/docs/refs-and-the-dom.html#callback-refs