make sure grid is set before trying to reinit tiles

git-svn-id: http://svn.openlayers.org/trunk/openlayers@908 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/WFS.js

@@ -131,7 +131,9 @@ OpenLayers.Layer.WFS.prototype =
         var newArguments = [upperParams];
         OpenLayers.Layer.Grid.prototype.mergeNewParams.apply(this, newArguments);
 
-        this._initTiles();
+        if (this.grid != null) {
+            this._initTiles();
+        }
     },
 
     /** combine the layer's url with its params and these newParams.