Next.js provides a router which can be utilized via a component imported via `next/link`, a wrapper `withRouter(Component)`, and now a hook `useRouter()`.
Invalid `href`s include external sites (https://google.com) and `mailto:` links. In the past, usage of these invalid `href`s could have gone unnoticed, but since they can cause unexpected behavior we now show a warning in development for them.
Look for any usage of `next/link` or `next/router` that is being passed a non-internal `href` and replace them with either an anchor tag (`<a>`) or `window.location.href = YOUR_HREF`.