Not passing `portalClassName` prop results in `<div class="undefined">`

[ffigiel]

Summary:

I noticed there's a bunch of silly <div class="undefined"> after upgrading my react-portal version.

Steps to reproduce:

1. Render a Modal without passing portalClassName prop
2. Observe that the portal div has a className of "undefined"
3. there's no 3

Expected behavior:

No class is given on the portal div.

Additional notes:

[ffigiel]

Hmm, this is a more specific case. I'm not yet sure what causes it. Here's how I render the modal:

class MyLegacyModalComponent extends Component {
    ...

    renderModal() {
        return (
            <Modal {...this.getModalProps()}>
                {this.renderModalContent()}
            </Modal>)
    }

    getModalProps() {
        return {
            className: this.getModalClassName(),
            isOpen: this.state.isModalOpen,
            onRequestClose: this.closeModal,
        }
    }

    ...
}

[ffigiel]

Dunno, it's probably something on my side. Closing.