AttributeError: 'NoneType' object has no attribute 'strip' about diff_cover HOT 4 CLOSED

# cobertura sometimes provides the sources for the measurements
        # within it. If we have that we outta use it
        sources = xml_document.findall('sources/source')
        sources = [source.text for source in sources]
        classes = [class_tree
                   for class_tree in xml_document.findall(".//class")
                   or []]

        classes = (
            [clazz for clazz in classes if
             src_abs_path in [
                 self._to_unix_path(
                     os.path.join(
                         source.strip(),
                         clazz.get('filename')
                     )
                 ) for source in sources]]
            or
            [clazz for clazz in classes if
             self._to_unix_path(clazz.get('filename')) == src_abs_path]
            or
            [clazz for clazz in classes if
             self._to_unix_path(clazz.get('filename')) == src_rel_path]
        )