Next Unhandled runtime error. No router instance found

I was making a portfolio in Next and i made just a header to show in all pages by putting it on _app.js, just that, never touched the _document.js. This is the error:

Unhandled Runtime Error Error: No router instance found. You should only use "next/router" inside the client side of your app. .next\static\chunks\fallback\main.js (116:10) @ getRouter

  114 |       'No router instance found.\n' +
  115 |       'You should only use "next/router" inside the client side of your app.\n'
> 116 |     throw new Error(message)
      |          ^
  117 |   }
  118 |   return singletonRouter.router
  119 | }