title: Mojofailureexception with Vaadin 20 and Webpack.js
body: <p>I've been trying to create a new app with vaadin-fusion (version 20), being an inexperienced student developer, I cannot resolve or even pinpoint the origin of the error that arises every time I attempt to compile frontend with vaadin designer (basically) every time the app is about to launch in Maven, this evidently, not a Maven-specific error, I attach the following stacktrace:</p>
  <pre><code>[ERROR] Failed to execute goal [com.vaadin:vaadin-maven-plugin:20.0.7:build-frontend(default-cli)on project myapp 
  Could not execute build-frontend goal
  Error occured during goal execution: There can only be one @PWA annotation and it must be set on the AppShellConfigurator implementor.Please run Maven with the -e switch (or Gradle with the --stacktrace switch), to learn the full stack trace. 
  [ERROR] 
  [ERROR] To see the full stack trace of the errors, re-run Maven with the -eswitch.
  [ERROR] Re-run Maven using the -X switch to enable full debug logging.
  [ERROR] 
  [ERROR] For more information about the errors and possible solutions, please read the following articles:
  [ERROR]  http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
  </code></pre>
  <p>The wiki link explains that maven error is caused by something else. The <code>@PWA</code> I haven't used at all. Perhaps, its a problem at the level of webcomponents module, which means its Vaadin's fault. Please, help has anyone encountered a similar problem? If yes, how did you resolve it?</p>