Index: trunk/src/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelper.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelper.java	(revision 16200)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelper.java	(revision 16201)
@@ -244,4 +244,6 @@
         changedKey = null;
         DataSet activeDataSet = OsmDataManager.getInstance().getActiveDataSet();
+        if (activeDataSet == null)
+            return;
         try {
             activeDataSet.beginUpdate();