“Warning: react-modal: App element is not defined. Please use `Modal.setAppElement(el)` or set `appElement={el}`”

Add ariaHideApp={false} to Modal attributes.

This should work:

<Modal isOpen={!!props.selectedOption}
    onRequestClose={props.clearSelectedOption}
    ariaHideApp={false}
    contentLabel="Selected Option"
    >
</Modal>