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;