diff -r aaa984499d36 -r ad063ac1f617 src/Pure/System/scala.ML
--- a/src/Pure/System/scala.ML	Sun May 24 10:36:42 2020 +0200
+++ b/src/Pure/System/scala.ML	Sun May 24 12:38:41 2020 +0200
@@ -32,6 +32,7 @@
 
 fun promise_function name arg =
   let
+    val _ = if Resources.is_pide () then () else raise Fail "PIDE session required";
     val id = new_id ();
     fun abort () = Output.protocol_message (Markup.cancel_scala id) [];
     val promise = Future.promise_name "invoke_scala" abort : string future;