Index: /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletionManager.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletionManager.java	(revision 3212)
+++ /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletionManager.java	(revision 3213)
@@ -109,4 +109,11 @@
 
     protected void cachePrimitives(Collection<? extends OsmPrimitive> primitives) {
+        if (tagCache == null) {
+            // We are coming from a DataSetListener event and
+            // rebuild has not been called yet, so do it now and 
+            // ignore the method parameter.
+            rebuild();
+            return;
+        }
         for (OsmPrimitive primitive : primitives) {
             cachePrimitiveTags(primitive);