In the document/README, it says the type of container returned from useCodeMirror is:

container: HTMLDivElement | null | undefined;

However, in the latest version (v4.19.9), the actual type that is exported from the npm package is:

container: HTMLDivElement | undefined;

I checked the source code, and I believe the type in document is right.

The source code

// https://github.com/uiwjs/react-codemirror/blob/6968faf74121748c629ab9487d5172beec9de42e/core/src/useCodeMirror.ts#L150

useEffect(() => setContainer(props.container!), [props.container]);

1. In the first render, the value of props.container is actually null which is the initial value of the ref editor in ReactCodeMirror.
   This change triggers a second render, because the new value null is different from the current value undefined.
2. Then in the second render, it can get the HTMLDivElement and pass it to container again, triggering a third render.
3. In the third render, the container finally becomes HTMLDivElement.

During the whole process, the container value is actually undefined -> null -> HTMLDivElement.