× error: element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined. you likely forgot to export your component from the file it
In my case (using Webpack) it was the difference between:
import {MyComponent} from '../components/xyz.js';
vs
import MyComponent from '../components/xyz.js';
The second one works while the first is causing the error. Or the opposite.