null check for notification removal

Make sure there is something to remove, before we attempt to remove the notifications. 
- This fixes #1240

modules/default/alert/alert.js

@@ -110,11 +110,13 @@ Module.register("alert",{
 	},
 	hide_alert: function(sender) {
                 //Dismiss alert and remove from this.alerts
+                if (this.alerts[sender.name]) {
                         this.alerts[sender.name].dismiss();
                         this.alerts[sender.name] = null;
                         //Remove overlay
                         var overlay = document.getElementById("overlay");
                         overlay.parentNode.removeChild(overlay);
+                }
 	},
 	setPosition: function(pos) {
 		//Add css to body depending on the set position for notifications