Good :)
When building a project (to get the meta-data needed to drive the editors in the workbench) we iterate all classes resolved from Maven and call the following:-
clazz.getDeclaredConstructors();
clazz.getDeclaredFields();
clazz.getDeclaredMethods();
clazz.getDeclaredClasses();
clazz.getDeclaredAnnotations();
Should any of those calls fail you'll get the error you report.
In short, there has to be a class somewhere that has a dependency on another class not in the Maven classpath.
What class this is: IDK. What dependency does this class have IDK either... all I can help with is explain where the error is coming from - leaving the research to you.
With kind regards,
Mike