AppMenu is now deprecated, and causes the app to fail to compile now. You can use Gtk.MenuButton now though :)
/messenger/src/HeaderBar.vala:24.13-24.32: error: The name `icon_widget' does not exist in the context of `Granite.Widgets.AppMenu'
app_menu.icon_widget = image;
AppMenu is now deprecated, and causes the app to fail to compile now. You can use Gtk.MenuButton now though :)