[FIX] mail: client action: crash on channel unsubscribe

After unsubscribing from a channel, the client action switches to inbox.
Method set_channel() takes as argument the channel to switch to, not its id.

addons/mail/static/src/js/client_action.js

@@ -462,7 +462,7 @@ var ChatAction = Widget.extend(ControlPanelMixin, {
         var self = this;
         chat_manager
             .unsubscribe(this.channel)
-            .then(this.set_channel.bind(this, "channel_inbox"))
+            .then(this.set_channel.bind(this, chat_manager.get_channel("channel_inbox")))
             .then(this.render_sidebar.bind(this))
             .then(function () {
                 var msg = _.str.sprintf(_t('You unsubscribed from <b>%s</b>.'), self.channel.name);