Index: trunk/src/org/openstreetmap/josm/gui/layer/WMSLayer.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/layer/WMSLayer.java	(revision 6496)
+++ trunk/src/org/openstreetmap/josm/gui/layer/WMSLayer.java	(revision 6497)
@@ -969,5 +969,5 @@
     @Override
     public void preferenceChanged(PreferenceChangeEvent event) {
-        if (event.getKey().equals(PROP_SIMULTANEOUS_CONNECTIONS.getKey())) {
+        if (event.getKey().equals(PROP_SIMULTANEOUS_CONNECTIONS.getKey()) && info.getUrl() != null) {
             cancelGrabberThreads(true);
             startGrabberThreads();