window: don't access inexistent header_bar

A HdyHeaderBar named header_bar does not exist in chatty-window.ui
leading to:

    (chatty:8445): GLib-GObject-CRITICAL **: 10:36:26.069: g_object_set: assertion 'G_IS_OBJECT (object)' failed
1 job for header-bar in 2 minutes and 2 seconds (queued for 2 seconds)