React: ‘Element type is invalid … check your render method’

Once you’ve seen this error once in your React app you’ll probably instantly recognize it and know exactly what this means:

Uncaught Error: Element type is invalid: expected a string 
(for built-in components) or a class/function (for composite 
components) but got: object. You likely forgot to export your 
component from the file it's defined in. Check the render 
method of `App`.

The key is “You likely forgot to export your component…” which was exactly the issue in my case.

If you have an ES6 style component this:

class App extends Component {
...
}

… check that you’ve exported it at the end of the source file with:

export default App;