Index: trunk/src/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelper.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelper.java	(revision 10131)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelper.java	(revision 10132)
@@ -589,5 +589,7 @@
                 cb.requestFocusInWindow();
                 cb.getEditor().selectAll();
-                sysSel.setContents(old, null);
+                if (old != null) {
+                    sysSel.setContents(old, null);
+                }
             } else {
                 cb.requestFocusInWindow();
Index: trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(revision 10131)
+++ trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(revision 10132)
@@ -203,5 +203,7 @@
                             Transferable old = Utils.getTransferableContent(sysSel);
                             editorComponent.selectAll();
-                            sysSel.setContents(old, null);
+                            if (old != null) {
+                                sysSel.setContents(old, null);
+                            }
                         } else {
                             editorComponent.selectAll();