The problem

After the upgrade react-modal, you can get the warning:

Warning: react-modal: App element is not defined. Please use Modal.setAppElement(el) or set appElement={el}. This is needed so screen readers don't see main content when modal is opened. It is not recommended, but you can opt-out by setting ariaHideApp={false}. 

Solution

When using the modal use ariaHideApp={false}, this will configure the model to not use the app.