src/Pure/PIDE/prover.scala

--- a/src/Pure/PIDE/prover.scala	Tue Sep 07 15:05:57 2021 +0200
+++ b/src/Pure/PIDE/prover.scala	Tue Sep 07 15:10:53 2021 +0200
@@ -302,7 +302,7 @@
 
   def protocol_command_raw(name: String, args: List[Bytes]): Unit =
     command_input match {
-      case Some(thread) if thread.is_active =>
+      case Some(thread) if thread.is_active() =>
         if (trace) {
           val payload = args.foldLeft(0) { case (n, b) => n + b.length }
           Output.writeln(