Index: trunk/src/org/openstreetmap/josm/gui/dialogs/relation/GenericRelationEditor.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/relation/GenericRelationEditor.java	(revision 17425)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/relation/GenericRelationEditor.java	(revision 17427)
@@ -865,7 +865,7 @@
         refreshAction.destroy();
         UndoRedoHandler.getInstance().removeCommandQueueListener(this);
+        super.dispose(); // call before setting relation to null, see #20304
         setRelation(null);
         selectedTabPane = null;
-        super.dispose();
     }