for fixed zoom layers, be careful that if the 3rd party tool is not loaded, they wont be able to return a valid getExtent() and therefore neither a valid getResolution(). Make sure those return null without crashing.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@1597 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/FixedZoomLevels.js

@@ -77,12 +77,18 @@ OpenLayers.Layer.FixedZoomLevels.prototype = {
      * @type float
      */
     getResolution: function() {
+        var resolution = null;
+        
         var viewSize = this.map.getSize();
         var extent = this.getExtent();
-        return Math.max( extent.getWidth()  / viewSize.w,
-                         extent.getHeight() / viewSize.h );
-    },
-    
+        
+        if ((viewSize != null) && (extent != null)) {
+            resolution = Math.max( extent.getWidth()  / viewSize.w,
+                                   extent.getHeight() / viewSize.h );
+        }
+        return resolution;
+     },
+
     /** Calculates using px-> lonlat translation functions on tl and br 
      *   corners of viewport
      *