From 76e8a800dacf59c9c7342cfaadca036083bc6fac Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=89ric=20Lemoine?= <eric.lemoine@gmail.com>
Date: Wed, 19 Oct 2011 23:47:25 +0200
Subject: [PATCH] special precautions must be taken in updateBackBufferData

---
 lib/OpenLayers/Layer/Grid.js | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lib/OpenLayers/Layer/Grid.js b/lib/OpenLayers/Layer/Grid.js
index fc2232a6a8..69584c2e97 100644
--- a/lib/OpenLayers/Layer/Grid.js
+++ b/lib/OpenLayers/Layer/Grid.js
@@ -490,10 +490,14 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
     updateBackBufferData: function() {
         // updateBackBufferData is called asynchronously when tiles are
         // received, so we need to check that the map is still there
-        if (this.map) {
+        if(this.map) {
             var resolution = this.map.getResolution();
             this.backBufferData.resolution = this.getServerResolution(resolution);
-            var topLeftTile = this.grid[0][0];
+        }
+        // the top left tile might have been destroyed, so its bounds
+        // property may be null
+        var topLeftTile = this.grid[0][0];
+        if(topLeftTile.bounds) {
             this.backBufferData.lonlat = {lon: topLeftTile.bounds.left,
                                           lat: topLeftTile.bounds.top};
         }