Fix the “Objects are not valid as a React child” error

I had this error in a React (Next.js) app:

Error: Objects are not valid as a React child (found: [object Promise]).
If you meant to render a collection of children, use an array instead.

After some time trying to figure out what the error meant, I figured out I was exporting my page component as async because I copied it from another Next.js project where this is possible because of the use of the app folder:

export default async function Page() {

}

But it was not possible in the pages folder.

Removing async made it work:

export default function Page() {

}