Index: trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Selector.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Selector.java	(revision 15918)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Selector.java	(revision 15920)
@@ -379,5 +379,7 @@
         private void visitBBox(Environment e, AbstractFinder finder) {
             boolean withNodes = finder instanceof ContainsFinder;
-            if (left instanceof OptimizedGeneralSelector) {
+            if (e.osm.getDataSet() == null) {
+                // do nothing
+            } else if (left instanceof OptimizedGeneralSelector) {
                 if (withNodes && ((OptimizedGeneralSelector) left).matchesBase(OsmPrimitiveType.NODE)) {
                     finder.visit(e.osm.getDataSet().searchNodes(e.osm.getBBox()));
@@ -438,4 +440,5 @@
                 e.parent = e.osm;
                 if (right instanceof OptimizedGeneralSelector
+                        && e.osm.getDataSet() != null
                         && ((OptimizedGeneralSelector) right).matchesBase(OsmPrimitiveType.WAY)) {
                     final CrossingFinder crossingFinder = new CrossingFinder(e);