FIX: hotfix for problems with the missing --prefix option

src/caoscrawler/crawl.py

@@ -1088,6 +1088,8 @@ def crawler_main(crawled_directory_path: str,
             if isinstance(elem, db.File):
                 # correct the file path:
                 # elem.file = os.path.join(args.path, elem.file)
+                if prefix is None:
+                    raise RuntimeError("No prefix set. Prefix must be set if files are used.")
                 if elem.path.startswith(prefix):
                     elem.path = elem.path[len(prefix):]
                 elem.file = None