Bug 1045919 - JavaScript Error: "TypeError: AppWindowManager.getActiveApp(...) is null" {file: "app://system.gaiamobile.org/js/activity_window_manager.js"

https://bugzilla.mozilla.org/show_bug.cgi?id=1045919
Well I guess it's one line work.

return this._activeApp || (homescreenLauncher ? homescreenLauncher.getHomescreen() : null);