An XML file starting with a DTD declaration such as
<!DOCTYPE art SYSTEM "article.dtd"> causes the XML-hul module to fail
and report a "Not well-formed" status:

  Status: Not well-formed
  ErrorMessage: SaxParseException: java.lang.ClassCastException: class sun.net.www.protocol.file.FileURLConnection cannot be cast to class java.net.HttpURLConnection(sun.net.www.protocol.file.FileURLConnection and java.net.HttpURLConnection are in module java.base of loader 'bootstrap'): SAXException cause: java.lang.ClassCastException
   ID: XML-HUL-3

This is caused by a failure in the XMLModuleHandler.resolveEntity method which fails to detect that the URL is a file type and then trys to cast the obj variable, a FileURLConnection instance, to HttpURLConnection causing the exception.

This branch has the beginings of a fix. This takes a simpler approach, rather than casting the URL instance it uses the URL.openStream method. It also catches any IOException and returns null, to "require default behaviour".

Reported by @samalloing