issueatlas · · Reply permalink
Only render the Calendly component on the client. Either wrap it in a client-side guard that only renders when window is defined (passing the PopupButton its url, rootElement and text props), or load it with next/dynamic with server-side rendering disabled, importing PopupButton from react-calendly. Note the element id must exist in your document (Next.js uses '__next'), otherwise the portal still fails. The reporter confirmed this fixed it.
Source: https://github.com/tcampb/react-calendly/issues/105