Index: /trunk/src/org/openstreetmap/josm/gui/widgets/QuadStateCheckBox.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/widgets/QuadStateCheckBox.java	(revision 12832)
+++ /trunk/src/org/openstreetmap/josm/gui/widgets/QuadStateCheckBox.java	(revision 12833)
@@ -238,5 +238,7 @@
         public void setEnabled(boolean b) {
             setFocusable(b);
-            other.setEnabled(b);
+            if (other != null) {
+                other.setEnabled(b);
+            }
         }