src/Pure/Thy/present.scala

     val dir = info_path + Path.basic(chapter)
     Isabelle_System.mkdirs(dir)
 
     val sessions0 =
       try { read_sessions(dir) }
-      catch { case _: XML.XML_Atom => Nil case _: XML.XML_Body => Nil }
+      catch { case _: XML.Error => Nil }
 
     val sessions = (SortedMap.empty[String, String] ++ sessions0 ++ new_sessions).toList
 
     write_sessions(dir, sessions)
     File.write(dir + index_path, HTML.chapter_index(chapter, sessions))