Index: /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(revision 10101)
+++ /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(revision 10102)
@@ -136,5 +136,7 @@
                 Transferable old = Utils.getTransferableContent(sysSel);
                 editorComponent.select(start, end);
-                sysSel.setContents(old, null);
+                if (old != null) {
+                    sysSel.setContents(old, null);
+                }
             } else {
                 editorComponent.select(start, end);