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The cause of the error message was that this line
import HelloWorld from '../js/Vue/assets/js/helloWorld.js?v=@version';
was expecting a default export, and none was found.
I tried
import { HelloWorld } from '../js/Vue/assets/js/helloWorld.js?v=@version';
but this named export was also not found.
I finally resolved this by publishing the "child" app as a library.
I still don’t know why the original method did not result in any exported module.
Source:stackexchange.com