diff -r c255ed582095 -r a5fda30edae2 src/Pure/Thy/export_theory.scala
--- a/src/Pure/Thy/export_theory.scala	Tue Apr 07 21:07:28 2020 +0200
+++ b/src/Pure/Thy/export_theory.scala	Tue Apr 07 21:49:36 2020 +0200
@@ -30,7 +30,7 @@
     store: Sessions.Store,
     sessions_structure: Sessions.Structure,
     session_name: String,
-    progress: Progress = No_Progress,
+    progress: Progress = new Progress,
     cache: Term.Cache = Term.make_cache()): Session =
   {
     val thys =