`NextRouter` was not mounted

A component used useRouter outside a Next.js application, or was rendered outside a Next.js application. This can happen when doing unit testing on components that use the useRouter hook as they are not configured with Next.js' contexts.

This can also happen when you try to use the useRouter hook from next/router inside the app directory, as the App Router's useRouter from next/navigation has different behavior to the useRouter hook in pages.

If used in a test, mock out the router by mocking the next/router's useRouter() hook.

If used in the app directory, migrate to the new hooks imported from next/navigation.