diff -r ef623f6f036b -r f373fb77e0a4 src/Pure/GUI/system_dialog.scala
--- a/src/Pure/GUI/system_dialog.scala	Tue Apr 22 23:31:45 2014 +0200
+++ b/src/Pure/GUI/system_dialog.scala	Tue Apr 22 23:49:15 2014 +0200
@@ -26,7 +26,7 @@
 
   private def check_window(): Window =
   {
-    Swing_Thread.require()
+    Swing_Thread.require {}
 
     _window match {
       case Some(window) => window
@@ -48,7 +48,7 @@
 
   private def conclude()
   {
-    Swing_Thread.require()
+    Swing_Thread.require {}
     require(_return_code.isDefined)
 
     _window match {