src/Pure/System/progress.scala

     override val verbose: Boolean = false
   ) extends Msg {
     override def show_theory: Msg = this
     override def message: Message = this
 
-    def output_text: String =
+    lazy val output_text: String =
       kind match {
         case Kind.writeln => Output.writeln_text(text)
         case Kind.warning => Output.warning_text(text)
         case Kind.error_message => Output.error_message_text(text)
       }