Index: trunk/src/org/openstreetmap/josm/actions/SimplifyWayAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/SimplifyWayAction.java	(revision 11486)
+++ trunk/src/org/openstreetmap/josm/actions/SimplifyWayAction.java	(revision 11488)
@@ -18,4 +18,5 @@
 
 import javax.swing.JOptionPane;
+import javax.swing.SwingUtilities;
 
 import org.openstreetmap.josm.Main;
@@ -55,10 +56,12 @@
 
     protected void alertSelectAtLeastOneWay() {
-        new Notification(
-                tr("Please select at least one way to simplify."))
-                .setIcon(JOptionPane.WARNING_MESSAGE)
-                .setDuration(Notification.TIME_SHORT)
-                .setHelpTopic(ht("/Action/SimplifyWay#SelectAWayToSimplify"))
-                .show();
+        SwingUtilities.invokeLater(() ->
+            new Notification(
+                    tr("Please select at least one way to simplify."))
+                    .setIcon(JOptionPane.WARNING_MESSAGE)
+                    .setDuration(Notification.TIME_SHORT)
+                    .setHelpTopic(ht("/Action/SimplifyWay#SelectAWayToSimplify"))
+                    .show()
+        );
     }