Fix list_view-related error in background page

Background page conversations were trying to trigger events on the inbox
list view which had been destroyed, resulting in a background page
console error of "can't read innerHeight of null".

Avoid this by removing listeners when the inbox window is closed.

js/views/inbox_view.js

@@ -35,6 +35,9 @@
             }).render();
 
             window.addEventListener('resize', this.inbox.resize.bind(this.inbox));
+            window.addEventListener('beforeunload', function () {
+                this.inbox.stopListening();
+            }.bind(this));
         },
         events: {
             'keyup': 'keyup',