Index: trunk/src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java	(revision 5868)
+++ trunk/src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java	(revision 5869)
@@ -361,4 +361,5 @@
      *
      * @param from  the source data set
+     * @param progressMonitor the progress monitor, can be {@code null}
      */
     public void mergeFrom(final DataSet from, ProgressMonitor progressMonitor) {
@@ -407,5 +408,8 @@
             Main.map.mapView.repaint();
         }
-        Main.map.conflictDialog.warnNumNewConflicts(numNewConflicts);
+        // warn about new conflicts
+        if (numNewConflicts > 0 && Main.map != null && Main.map.conflictDialog != null) {
+            Main.map.conflictDialog.warnNumNewConflicts(numNewConflicts);
+        }
     }