Error: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: number. code example

Example 1: element type is invalid expected a string

In my case (using Webpack) it was the difference between:

import {MyComponent} from '../components/xyz.js';
vs

import MyComponent from '../components/xyz.js';

Example 2: × 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.