Index: src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java
===================================================================
--- src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java	(revision 14478)
+++ src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java	(working copy)
@@ -1053,6 +1053,8 @@
         super.destroy();
         data.removeSelectionListener(this);
         data.removeHighlightUpdateListener(this);
+        if (!data.isLocked())
+            data.clear();
     }
 
     @Override