Bug 820799 - Set the lastSelected property to an empty string rather than null, since the latter stringifies to 'null'. r=neil

--HG--
extra : rebase_source : 18301071202f5d2f50d5d8cd4c06eab7ab03b155

toolkit/content/widgets/preferences.xml

@@ -629,7 +629,7 @@
         if (this.lastSelected) {
           lastPane = document.getElementById(this.lastSelected);
           if (!lastPane) {
-            this.lastSelected = null;
+            this.lastSelected = "";
           }
         }