Objects are not valid as a React child (found: [object Promise])

I also received the same error message when creating an async functional component. Functional components should not be async.

const HelloApp = async (props) =>  { //<<== removing async here fixed the issue
  return (
    <div>
      <h2>Hello World</h2>
    </div>
  )
}
ReactDOM.render(<HelloApp />, document.querySelector("#app"))

jsfiddle