Bug 909640 - Fix js warnings in shell.js r=gwagner

b2g/chrome/content/shell.js

@@ -757,6 +757,9 @@ var AlertsHelper = {
             return helper.resolveFromOrigin(message["notification"]);
           }
         }
+
+        // No message found...
+        return null;
       }
 
       listener.target = getNotificationURLFor(manifest.messages);
@@ -833,7 +836,7 @@ var AlertsHelper = {
     if (!aMessage.target.assertAppHasPermission("desktop-notification")) {
       Cu.reportError("Desktop-notification message " + aMessage.name +
                      " from a content process with no desktop-notification privileges.");
-      return null;
+      return;
     }
 
     let data = aMessage.data;