When does enqueueSnackbar return null?

[Svish]

I'm creating a wrapper for useSnackbar with some defaults and additions in our project, and since we're using Typescript I want the typings to be correct. And in the notistack typings there's a thing I don't quite understand:

The enqueueSnackbar function is typed to return OptionsObject['key'] | null, while closeSnackbar can only accept OptionsObject['key'].

In what sort of cases does enqueueSnackbar return null? 🤔

export interface WithSnackbarProps {
    enqueueSnackbar: (message: SnackbarMessage, options?: OptionsObject) => OptionsObject['key'] | null;
    closeSnackbar: (key?: OptionsObject['key']) => void;
}

https://github.com/iamhosseindhv/notistack/blob/master/src/index.d.ts#L35-L38

I've tried to figure it out from the documentation and looking in the source, but couldn't really find it... 😕

[iamhosseindhv]

This is incorrect type in index.d.ts.
We used to return null when preventDuplicate props was set to true.

As mentioned here, as of #189 enqueueSnackbar no longer return null.

Fix will be published in v0.9.8.