Index: /trunk/src/org/openstreetmap/josm/data/osm/QuadBuckets.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/osm/QuadBuckets.java	(revision 2910)
+++ /trunk/src/org/openstreetmap/josm/data/osm/QuadBuckets.java	(revision 2911)
@@ -648,4 +648,7 @@
                 nr_siblings--;
             }
+            if (nr_siblings == 0) {
+                parent.children = null;
+            }
             if (parent.canRemove()) {
                 parent.remove_from_parent();