null check for notification removal

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

modules/default/alert/alert.js

@@ -109,12 +109,14 @@ Module.register("alert",{
 
 	},
 	hide_alert: function(sender) {
-		//Dismiss alert and remove from this.alerts
-		this.alerts[sender.name].dismiss();
-		this.alerts[sender.name] = null;
-		//Remove overlay
-		var overlay = document.getElementById("overlay");
-		overlay.parentNode.removeChild(overlay);
+                //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