recommitting r574 - the bug was in the OpenLayers.Feature destroy() function, which was not checking to ensure a valid map setting.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@576 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/WFS.js

@@ -47,6 +47,14 @@ OpenLayers.Layer.WFS.prototype =
     },    
     
 
+    /**
+     * 
+     */
+    destroy: function() {
+        OpenLayers.Layer.Grid.prototype.destroy.apply(this, arguments);
+        OpenLayers.Layer.Markers.prototype.destroy.apply(this, arguments);
+    },
+    
     /** 
     * @param {OpenLayers.Bounds} bounds
     * @param {Boolean} zoomChanged